ARISTOTLE ON WHAT IS DONE IN PERCEIVING

Theodor Ebert

I.

Sense perception in Aristotle’s psychology is conceived of as ablend of activity and passivity.1 Some people think that Aristotle allows passivity too great a share,2 but I shall, in what follows, not be concerned with what Aristotle says about the passive rôle of the senses; what I want to discuss is the active side to sense perception. This active side is most commonly thought of as a judgmental activity.
The Greek word which is supposed to express this concept of judging and judgment is the verb κρίνειν; κρίνειν and derivative and composite forms (κριτικός, κριτής, ἐπικρίνειν, διακρίνειν – the latter only in the Parva Naturalia, not in the De Anima) do not make particularly significant entries in the indexes of Aristotle’s psychological writings.3 It is not their frequency but the contexts in which they are used that secures a key rôle to κρίνειν and related forms: virtually a third of all the occurrences in the De Anima is to be found in the last section of book III chapter 2 (426b8-427a16) where Aristotle raises questions that eventually lead up to his doctrine of the common sense; τὸ κρÃνον is used here (427a11) as a label für what is to be called the κύριον αἰσθητήριον in the Parva Naturalia (De Somno 2, 455a21), and what has traditionally been referred to as the sensus communis, the ‘common sense’. In the Parva Naturalia, Aristotle when discussing the sensus communis invariably falls back on the κρίνειν – terminology (cf. De Somno 2, 455a12-20; De Insomn. 2, 460b16-17; 3, 461b3-7, b25). Again, when thinking is compared or contrasted with perceiving in De Anima III 3 and 4 (esp. 427a19-21, 428a1-4, 429b10-22), κρίνειν is brought in to illuminate similarities and differences between the two.
Κρίνειν is indeed the term used to describe what is done when somebody is perceiving or thinking, in opposition to what may be undergone on these occasions. It is, therefore, of same importance to know what sort of activity Aristotle wants to denote by this expression. If we are justified in translating κρίνειν by ‘to judge’, κριτικόν by ‘faculty of judgment’ and so on, then Aristotle, as far as his theory of mind and of perception in partieular is concerned, seems to be quite close to philosophers of the last few centuries. Talk of judgment in connection with sense perception has, after all, a easy Kantian ring.	
In order to see the issue involved more dearly, let us first take a look at some translations and commentaries of the De Anima. There seems to be, in fact, considerable disagreement among them as to how κρίνειν and related expressions should be rendered. Professor Hamlyn,4 in his (partial) translation of the De Anima, uses indeed ‘to judge’ für κρίνειν and cognate expressions on all occasions but one (431a20, where he translates ἐπικρίνειν by ‘to determine the difference’). We find a similar picture in the recent French translation by E. Barbotin,5 who has ‘juger’ on all but three occasions, and also in the translation included in Hicks’ commentary;6 Hicks makes use of the ‘judgment’ terminology in the great majority of cases, and talk of ‘judgment’, ‘passing judgments’ and ‘faculty of judgment’ bulks very large in his commentary, too.
But as only a glance at a Greek dictionary will tell us, a very common meaning of κρίνειν, at least in fourth century literature, is that of ‘discerning’ and ‘discriminating’. And the ‘discerning’ /‘discriminating’ terminology has found at least as much favour with translators and commentators of the De Anima as the ‘judgment’ vocabulary. Both Barbotin and Hicks use it occasionally, and the French commentator G. Rodier, in the translation included in his commentary,7 favours ‘discerner’ with a very few exceptions (once he has ‘saisir’ – 404b25 – and three times he uses ‘juger’: 411a4, a6, 431a20). J. A. Smith, the Oxford translator,8 has ‘to discern’ or ‘to discriminate’ in the overwhelming majority of cases and avoids the judgmental terminology altogether. Sir David Ross,9 in the rendering of the text preceding his comments on the separate chapters, is less consistent than J. A. Smith – he uses ‘to judge’ twice (on 418a14 and 428a3) – but apart from those cases he sticks to the ‘discerning’/‘discriminating’ vocabulary, too. Other translators (Wallace, Hett, the German translator W. Theiler, and the medieval translator William of Moerbeke)10 have a mixture of both candidates; the same, incidentally, is true of the translations of the Parva Naturalia by Mugnier11 and by Beare and Ross12 and of Sir David Ross’s renderings in his Parva Naturalia commentary.13
Thus, we just seem to be left with a confusing disagreement among translators, as far as κρίνειν and cognate expressions in Aristotle’s psychological writings are concerned. There is, however, a more specific observation to be made on the different commentators we have come across. For although there is, in fact, one English translation of the De Anima (J. A. Smith) which does not make use of the judgmental terminology, none of the commentators mentioned which use ‘to discern’/‘to discriminate’ for κρίνειν on same or most occasions avoids use of ‘to judge’, ‘to pass judgment’ etc. completely. Professor Hamlyn, on the other hand, who refrains from using the ‘discern’/‘discriminate’ terminology in his translation, is less consistent in his commentary: in his remarks on the De Anima III 2, he fenders the participle τὸ κρÃνον in 427a11 by “that which judges or discriminates”.14 This observation (and, in particular, such twin renderings as the one quoted from Hamlyn’s commentary) strongly suggests that the commentators mentioned do not see a philosophically important difference between judging on the one hand and discerning/discriminating on the other.15 One is left with the impression that for them the term to choose is more a question of stylistic expediency than of conceptual clarity.
I think that there is indeed a philosophically important difference between judging and discerning/discriminating. Even on a superficial inspection the two groups of expressions present dissimilarities sufficient to make us wonder which, on any occasion, might be the correct translation. ‘To discern’/‘to discriminate’ are success verbs, but misjudging is still a form of judging. You can discern or discriminate something clearly, sufficiently or insufficiently, but not truly or falsely; whereas you can judge truly or falsely, but hardly clearly or insufficiently: ‘judging’ is a propositional verb, ‘discerning’ and ‘discriminating’ are cognitive verbs. Your ability to discern or discriminate something may depend on and, what is more, may vary with certain circumstances; this can hardly be said of your ability to form a particular judgment: ‘discerning’/‘discriminating’ denote achievements that can go on for a time, i. e. are retentions; judging, when designating an act not a disposition, does not extend over a stretch of time. Discerning and discriminating may involve an effort, judging hardly can. You may judge i. e. form or pass your judgment on the basis of evidence or information or observation, or even on the basis of other judgments; but it does not make sense to say that you discern something or discriminate something from something on the basis of something else: discerning and discriminating are themselves radically basic cognitive activities, judging cannot be said to have a similar position in the field of propositional acts (although certain kinds of judging may be in such a position).
One point immediately emerges from the foregoing observations: most of the features which are characteristic of discerning/discriminating are shared by perceptual activities, like seeing, hearing, observing, and listening, but also smelling, tasting, and feeling; all of these activities are not enacted on the basis of something else, of some other activity or result of activity; all of these activities may go on for a time, and one’s ability to perceive something in the ways just mentioned depends on and may vary with the circumstances; even in the case of tasting and feeling, where the last-mentioned feature is perhaps not as obvious as in other cases, our ability to exercise these faculties may depend on temperature.
There is, however, a difference between those verbs of perception which designate perceptual activities presupposing only the exercise of the corresponding faculty (like ‘to see’, ‘to hear’, ‘to smell’ etc.) and those verbs which denote activities implying the use of perceptual faculties for a specific end (like ‘to observe’ or ‘to listen’). Only activities of the latter sort may be said to involve an effort. Thus, discriminating and discerning go together with those perceptual activities that imply the use of a faculty in contradistinction to its mere exercise. Discriminating and discerning are, indeed, themselves activities which imply the use of a faculty, although they are not restricted to a specific faculty, as e. g. observing seems to be tied to our visual, listening to our auditive faculty. They are not even restricted to the range of perceptual faculties: you can discriminate two concepts from each other, as well as two colours.
The foregoing observations should have made it clear that the difference between the two translations of the Greek κρίνειν we have come across is a philosophically important difference. In the following paper I shall try to decide the question as to how the word is to be understood in Aristotle’s psychological writings. Deciding this question will shed some new light on Aristotle’s theory of perception.

II.

Bonitz, in his Index to the Bekker edition, distinguishes a primary meaning of κρίνειν (which he does not label such, however), corresponding grosso modo to the ‘discerning’/‘discriminating’ sense, from what he calls a sensus iudicialis. Since κρίνειν in this latter sense seems to be a promising candidate for a Greek equivalent of the English ‘to judge’, let us start with a discussion of its use in the language of the law. The Greek verb is indeed used to refer to the activity of judges and is not what judges do just judging?
Let me first make a point about two ways in which the English ‘to judge’ can be used: you can judge a case and you can judge a man; if you judge a man, you pass judgment upon him and this may mean that you declare him guilty or innocent. Judging a case, however, means deciding it. When judging a case, you do not pass judgment upon the case. A noun like ‘man’ or ‘person’, on the other hand, could hardly go as an accusative with ‘to decide’.
What I want to claim now is that κρίνειν in the judicial sense (and any meanings dependant on it) is ‘judging’ only in the sense of ‘judging a case’. In short κρίνειν, in the judicial (and any related) sense means ‘deciding’. The Greek has two terms for ending legal proceedings by passing a verdict, δικάζειν and κρίνειν. Although it seems these two expressions are used indiscriminately in non-technical language, there is a clear distinction between them in the language of law proper: δικάζειν is used whenever the outcome of a litigation is, as it were, clear from the very beginning and the judge only spells out the application of the law to the particular case. Κρίνειν, however, is the term used in those cases which leave room for judicial discretion, either because there is no formally established evidence at hand or because the law provides that in such and such a case the decision is up to the judge anyway.16
This connection, in the technical language of Greek civil law, between the act denoted by κρίνειν and the discretional power of judges is, I think, very strong evidence to the effect that κρίνειν, as a term of law, means ‘to decide’. That it keeps this meaning, when taken over from the language of law to other contexts, can be seen from Aristotle’s Politics where the term is frequently used for decision-making quite generally. Thus, Aristotle recommends that the denomination ‘magistracy’ (ἀρχή) should be reserved for those offices which are given the power to hold deliberations as well as to take decisions and to issue orders (the respective Greek terms are βουλεύσασθαι, κρÃναι, ἐπιτάξαι Pol. IV 15, 1299a25-27). In another passage the citizens assembled are said to pass verdicts (δικάζουσι), to hold deliberations (βουλεύονται), and to take decisions (κρίνουσιν) (Pol. III 15, 1286a27f.). Here the use of κρίνειν in opposition to δικάζειν which retains its judicial sense is evidence for the quite general concept it is meant to convey.
The proposed way of understanding κρίνειν in the ‘deciding’ sense is further confirmed by the specific grammatical constructions of this ward. Leaving aside the somewhat rare constructions with an internal accusative (e. g. θεμίστας, [to issue] verdicts Il. XVI, 387; Hesiod, Erga 221 cf. Theog. 85; κρίσιν [to make] a decision Plato, Rep. II, 360e), its most common accusative objects are expressions like νεÃκος (strife, litigation e. g. Od. XII, 440; XVIII, 264) or δίκη (law-suit e. g. Plato, Legg. IX, 877b; Aristotle, Pol. III 14, 1285b11). But as in English we do not only talk of deciding a case, but may also say that someone or something (e. g. some act or event) decides a dispute, a competition, a debate or even the course of a disease, so we do in Greek. The Greek equivalents of these expressions are all to be found as accusatives of κρίνειν (ἔλεγχον Parm. B 1, 36; ἀγäνα Aristoph., Ranae 873; συμβουλάς Arist., Rhet. II 1, 1377b22; νόσους [Pseudo-]Aristotle, Probl. I 3, 859a10, a21).
Secondly, κρίνειν in the judicial sense goes with a set of prepositions, quite as the English ‘to decide’: you decide upon certain matters or about a person, according to certain rules or beliefs, and on the basis of e. g. certain evidence: the corresponding Greek prepositions are περί  with the genitive, κατά  with the accusative, and ἐκ with the genitive.17 Finally, that which has been decided is quite often expressed by means of a that clause; the Greek, for the corresponding construction, uses the accusative with an infinitive (esp. frequent in Plato, cf. Rep. IX, 580d; Gorg. 452c4-5; Theaet. 170d7-8; Phil. 67b3-4).
Before leaving the judicial sense of κρίνειν, we should notice two usages of this ward in judicial contexts which do not fit our picture of parallel constructions in Greek and English all that nicely. In opposition to its English counterpart, the Greek verb can be accompanied by accusatives referring to persons and, as any transitive verb, it can be used in the passive voice with a personal subject. In neither of these cases, however, does the word have the meaning ‘to pass judgment upon somebody’. It can either mean: ‘to try somebody’, ‘to put on trial somebody’ (ἄνδρα περὶ θανάτου κρίνειν to try a man for life Gorgias B 11a; κρίνεσθαι περὶ θανάτου to be tried for life Arist., Rhet. II 20, 1393b24),18 or else it can mean: ‘to decide in favour of someone’ (e. g. Arist., Soph. El. 25, 180b26-27; cf. also Liddell/Scott/Jones s. v. κρίνειν II. 7).
To conclude: the use of κρίνειν in judicial contexts does not offer any support for translating this verb by ‘to judge’ in the sense of ‘to pass judgment upon someone’. Obviously, the discerning/discriminating usage of the word cannot justify this translation either. Thus it is simply a mistranslation to render the Greek κρίνειν by ‘to judge’ in contexts where the English word has the force of ‘to contend’, ‘to assert’, as is the case in translations of Aristotle’s psychological writings. It is worth noticing that κρίνειν does not occur as a term for ‘judging’, ‘asserting’ in Aristotle’s Organon. Aristotle’s term used to express this concept is ἀξιοØν (cf. An. Priora I 17, 37a10, a20; I 24, 41b10, b 17; I 33, 47b28; An. Post. II 4, 91a37).19
What about the primary sense of κρίνειν, the one corresponding to discerning and discriminating? The underlying idea seems to be that of singling out and of separating by singling out; in this sense the word is used several times in the Homeric poems.20 Although κρίνειν retains its sense of selecting or choosing after Homer (e. g. Herod. 6, 129; Arist., Pol. II 9, 1271a22), its use as a term for discerning or discriminating is far commoner in later authors: its objects can be sense perceptual qualities (e. g. cold/warm, hard/soft, light/heavy in Xen., Hiero 1, 5) or else more abstract entities: one’s duty (ἃ δεÃ πράττειν Xen., Mem. IV, 1,4) or the truth (τἀληθές Anaxag. B 21; Plato, Theaet. 150b; Arist., De Caelo IV 10, 279b12). The Greek, however, does not use a preposition corresponding to ‘from’ or ‘between’ when κρίνειν has the sense of ‘separating’ or ‘discriminating’, it simply uses an ‘and’ (καί); this, of course, makes the two usages of ‘to discern’ (‘to select’), on the one hand, and ‘to discriminate’ (‘to separate’), on the other, much closer to each other than their respective equivalents in English or German.
Are there any grammatical features peculiar to this verb in the sense under discussion that would be helpful to distinguish a use of κρίνειν in this sense from one in the other sense, that of deciding? First of all, κρίνειν in the discerning/discriminating sense has as grammatical objects most commonly nouns referring to objects proper and their properties, especially their sense perceptual properties; it could not go with accusatives that designate processes. Secondly, κρίνειν in the sense under discussion may be construed with the preposition κατά c. acc. (as can its judicial counterpart), but here the accusative following κατά refers to the principle according to which the discrimination or separation is made, not to a principle according to which a decision is reached; it refers to a criterion rather than to rules or beliefs (cf. e. g. Il. II, 362). On the other hand, περί with the genitive or ἐκ with the genitive, i. e. those prepositions typical for the deciding sense, would hardly suit the discerning/discriminating usage of this word: one does not discern upon something, but something, nor do we discern or discriminate on the basis of something.
The most typical construction for κρίνειν in its sense of discerning/discriminating is the use of the instrumental dative; and the expressions that accompany κρίνειν in this case are either words for organs or tools or just faculty words. It is indeed quite a natural idea that discerning and discriminating imply the exercise of certain perceptual or intellectual faculties or the use of organs or tools in the exercise of such faculties. In the case of deciding, however, the idea of making use of faculties and/or organs or tools is much less natural; deciding may presuppose special powers, but rarely special faculties.21

III.

Let us turn back to Aristotle. In what sense are κρίνειν and derivative and composite forms used in the De Anima and in the Parva Naturalia (up to the De Divinatione)? It would be tedious to go through all occurrences of these expressions here and, consequently, I shall just state that, by applying the criteria established above, κρίνειν and cognate expressions will be found to have or (in the case of derivatives) to be connected to the sense of ‘to discern’/‘to discriminate’ on all but four occasions. In particular, the word is used in the discerning/discriminating sense throughout De Anima III. The main evidence for this claim is the use of instrumental datives referring to faculties and the use of faculty words as subjects in the overwhelming majority of cases. Where κρίνειν is not used in this way, its grammatical objects are expressions denoting sensible qualities (e. g. τὰ αἰσθητά 424a5, λευκὸν καὶ μέλαν 431a25), not words referring to processes.
To turn to the four exceptions: the least interesting is the one in the De Insomniis 3, 461a25 where διακρίνεσθαι is applied to the physiological process of the distribution of blood during sleep. Of comparably little interest is the use of κριτής in the De Anima I 2, 405b8: each of the four elements except earth has found a κριτής, i. e. someone deciding in its favour – a usage obviously connected to the somewhat special sense of κρίνειν we have come across earlier on; fortunately, all translators are agreed on the meaning of this word.
The two remaining cases are one in the De Anima II 6, the other one in the De Insomniis chap. 2. In both cases, the grammatical constructions involved indicate that κρίνειν is to be understood in the sense of ‘to decide’. In the De Insomniis passage Aristotle makes, with regard to the experiment known as ‘Aristotle’s illusion’, the counterfactual assumption that if we had no other sense than touch, we would decide (“judge”) that one is two (εἰ δ’µν ἡ ἁφὴ μόνη, κἂν ἐκρίνομεν τὸ ἓν δύο [sc. εÉναι] 460b22). Although this claim occurs within the discussion of sense perception, the use of the accusative with an infinitive construction is, I think, overriding evidence in favour of the deciding sense of κρίνειν: whenever κρίνειν in the other sense is used with a that clause – we shall come to some cases lateron – it goes with a ὅτι.
The last example occurs in De Anima II 6; Aristotle there discusses three sorts of αἰσθητά (or perhaps three meanings of the Greek term), and the first to be discussed are the special sensibles, the sense objects proper to each of the five senses; thus, colour belongs to sight, sound to hearing and so on (418a13). After this remark, the text continues:

“But each sense decides about (or: is the judge of) these objects (κρίνει περὶ τούτων i. e. the special sensibles) and makes no mistake about their being respectively colour and sound, but only about what or where the coloured or the sounding object is.” 					       (418a14-16)

The use of the preposition περί with the genitive makes it clear that κρίνειν, in this passage, is to be taken as ‘deciding about’. What is more, this translation of our passage renders it more meaningful than the rival translation ‘discerns’ (Wallace, Smith cf. the French translation in Rodier) or ‘judges of’ (Hicks, Ross) or ‘judges about’ (Hamlyn). Aristotle wants to bring out features peculiar to the relation between the senses and their special sensibles; now, it certainly is not a peculiar feature of this relationship that the senses discern (or judge about) their special sensibles – that would also hold good for their relation to e. g. the common sensibles. On the proposed reading, Aristotle, however, makes each sense, as it were, sit as a judge, an arbiter upon its corresponding special sensible. The verb κρίνειν is meant not so much to characterize an activity proper – for we or our senses do not make decisions about what the world looks like, at any rate not according to Aristotle – but to describe the position of the senses vis-à-vis their corresponding special sense objects. And they are not in a similar position if it comes to the common sensibles; for there it is always possible to appeal to another sense against the verdict of the first.

IV.

Now, granted that all I have said so far on philological questions is correct, that is, granted that κρίνειν has in fact been mistranslated in key passages of Aristotle’s psychological writings when rendered by ‘to judge’ and cognate expressions where ‘to discern’/’to discriminate’ would be appropriate, is there anything philosophically important about correcting this mistranslation? Is there any philosophical insight on the part of Aristotle that has been obscured by this faulty rendering? In the rest of this paper I shall try to give an affirmative answer to this question. In particular, I want to show that Aristotle had very good reasons to use the concept of discerning/discriminating as a fundamental one in the analysis of perception.
Let me start with some preliminary remarks: From what has been said about the conceptual difference between judging in the one hand and discerning/discriminating on the other it should not be inferred that Aristotle wants to deny the rôle of propositional attitudes in sense perception. Perceptual activities are quite often undertaken in order to test, to refute or to establish a belief, and beliefs certainly are propositional attitudes. Most perception verbs, in Greek as well as in English, can be used not only with a direct object but also with a that clause.22
Aristotle repeatedly states that perception of the special sensibles is always true (De Anima II 6, 418a15-16; III 3, 427b11-12; 428b18-19; III 6, 430b29; De Sensu 4, 442b8-10). Perception of the special sensibles is attributed to all animals (De Anima III 3, 427b12-13). Animals are also said to be deceived, even for most of the time (De Anima III 3, 427a29-b2), probably because of errors about the common sensibles, or about what is perceived incidentally (De Anima III 3, 428b20-22; De Sensu 4, 442b9-10). We find Aristotle using αἰσθάνεσθαι as well as κρίνειν with a ὅτι-clause in two passages, one in the De Anima (III 2, 426b12-23) and one in the De Somno (2, 455a16-18). Thus, we may assume that Aristotle was aware of the close connection of sense perception and propositional attitudes. Truth and falsity, after all, presuppose propositions.
There is, however, a difficulty here. Aristotle states that brutes do not have λόγος (De Anima III 3, 428a23-24). But brutes do certainly partake in perception of special and common sensibles. Thus, if λόγος is meant to designate a faculty presupposed in forming propositions or in having propositional attitudes, we are landed with a flat contradiction. I shall argue that this difficulty is only apparent; λόγος, when used as a faculty word, is not dependent on the use of this expression referring to statements or propositions (as e. g. in De Interpretatione 4, 16b26-28 cf. chap. 5 and 6), but on the use of this word as referring to arguments (as e. g. in Topics VIII 12, 162a35; 162b3, 16). The Greek λόγος, when designating a faculty, simply means ‘reason’ in the strict sense of the reasoning faculty. It is a faculty which enables us to see consequences and to draw conclusions; and that is something animals do lack.
That Aristotle uses λόγος in this sense of ‘reason’, i. e. ‘reasoning faculty’ can be seen from a passage in the De Anima III:

“Every opinion implies conviction (πίστις), conviction implies that we have been persuaded, and what persuades is reason (λόγος).” 
De Anima III 3, 428a22-23

What is presupposed on the part of somebody we want to persuade is his ability to understand the (good or bad) arguments we may use to convince him of something. He must be capable not only to understand the information conveyed, but also the implications entailed in a statement or in a conjunction of statements.
Further evidence for this sense of λόγος, when used as a faculty word, is the close connection of this concept with διάνοια and διανοεÃσθαι: λόγος is said to be a necessary condition for διανοεÃσθαι. (De Anima III 3, 427b 13-14). And from an example used in the Analytica Priora (I 15, 34b32-36) it is clear that διάνοια cannot be predicated of a crow, and hence, we may infer, not of animals (in contradistinction to men) in general. It can, as the same passage shows, be predicated of every man. Thus, both faculties, λόγος as well as διάνοια seem to be specific properties of man, closely related to each other. But διάνοια is a faculty enabling us to foresee future events, i. e. to recognize consequences of past or present states of affairs. (Pol. I 2, 1252a32). We may therefore safely assume that Aristotle, when he denies λόγος to animals, does not want to deny them the faculty of having propositional attitudes, but only the reasoning faculty, the faculty to realize implications and to draw inferences.
Now Aristotle, as we have seen above, allows for truth and error in sense perception quite generally i. e. including sense perception of animals. The conclusion to be drawn from this can only be that he allows for propositional attitudes in animals as well as in men. Why is this point worth stressing? After all, as has been shown at the outset, discriminating/discerning is an activity which should be kept conceptually distinct from judging. But now we have to bring in another point as well; for, although κρίνειν does not mean judging and does not designate same kind of judging either, the cognitive activity denoted by this word results in judgments and propositional attitudes. And the important thing about the beliefs and propositional attitudes generated by this cognitive activity is that, at their most basic level, these propositional attitudes correspond to propositions of a specific kind: to put it briefly – the corresponding propositions are not of the copulative, subject-predicate form, but are relational ones.
To make this point more explicit, we may start from same observations about the logic of the verb ‘to discriminate’; I shall concentrate on ‘to discriminate’ to the exclusion of ‘to discern’ for the reason that discriminating seems to be the more fundamental concept: discerning is discriminating something from a background. Aristotle uses κρίνειν when it denotes a cognitive activity, almost exclusively in the discriminating (as opposed to the discerning) sense: if construed with accusative objects, the verb usually goes together with two objects connected by καί (cf. De Anima III 2, 426b10f., b17; III 7, 431a20, a24; De Sensu 1, 436b15; 7, 447b27; De Memoria 452b8).
‘To discriminate’ is a three-place predicate and if we take 
‘S discriminates x from y
as the canonic formula, the relation expressed by this predicate is symmetrical as to the second and third place: clearly, if someone discriminates a from b, then it is true to say that he discriminates b from a as well. In the above formula, ‘S’ is a variable ranging over subjects of cognitive activity, ‘x’ and ‘y’ are variables ranging over objects of such activities. The relation existing between instances of ‘x’ and ‘y’ is irreflexive (it would be logically improper to say “he discriminates a from a”) and a-transitive (neither transitive nor intransitive). Now, these three logical features – symmetry, irreflexivity, and a-transitivity – characterize the relation of ‘being different from’. Aristotle, therefore, quite naturally connects the concept of discrimination and that of difference or being different from (cf. De Anima III 2, 426b10, b 17-18; III 7, 431a20; De Somno 455a17-18).
I said that (in the above formula) ‘S’ ranges over subjects of cognitive activity. That is, however, ambiguous. It can be taken to mean ‘subjects of cognitive behaviour’ i. e. men and animals. But we can also use expressions like ‘sight’, ‘hearing’ or ‘the eye’ or ‘the mind’ as subject expressions going with ‘to discriminate’, expressions, that is, which designate organs or faculties. If the subject expression used refers to men or animals, it is always possible to supplement the sentence by bringing in an instrumental dative referring to an organ or a faculty. Aristotle, in his analysis of sense perception, is very careful either to use faculty words as subjects of κρίνειν or to put in an instrumental dative containing or standing for a faculty word (cf. De Anima I 2, 404b25-27; II 10, 422a21; III 2, 425b21, 426b8-10, b12-15; III 4, 429b14-15; De Sensu 1, 436b15-16; 6, 445b15; 7, 447b26-448a1; De Memoria 2, 452b8).
I think that Aristotle had excellent reasons for being so insistent on using κρίνειν together with faculty words. For consider a situation where the statement “The car over there is green” is a correct report of an actual observation. The person uttering these words is able to see the car and to see its colour, and he is able to distinguish car from colour, to discriminate the thing perceived and its property. But whereas the car and its colour are perceived by the sense of sight (although perhaps not in the same sense of ‘to perceive’), the difference between the two is nothing that could be detected by our visual faculty. Thus, if you put an expression designating a sense in the place of the grammatical subject of ‘to discriminate’ (or if you use the appropriate instrumental dative), you restrict the range of expressions that can serve as instances for ‘x’ and ‘y’ in the formula ‘S discriminates x from y’. The expressions allowed are subject to the following condition: it must be possible to perceive the difference of the two sensible objects (using that expression in the wide sense of αἰσθητά) designated by the expressions used in a statement of discrimination by the sense that perceives the sensible objects themselves.
This condition is fulfilled in cases where you discriminate colours by sight, sounds by hearing, and flavours by taste, standard Aristotelian examples (cf. De Anima III 2, 426b8-12; III 7, 431a24-25). It would, however, be wrong to say that the special sensible objects of a particular sense perceptual faculty can always be put in this relation: the sense of touch is the important counter-example. You do not discriminate the temperature of an object from the smoothness of its (or another thing’s) surface by the sense of touch. In contradistinction to sight, hearing, and taste, touch embraces several types of difference, as Aristotle is careful to point out (De Anima II 6, 418a12-14).In fact, we do not use our fingers in the same way when trying to find out about something’s temperature, its surface qualities like smoothness or roughness, and its material qualities like softness or hardness: we merely touch it in the first case, we move our fingers in the second, and we press against it in the third. Thus, the sensible objects to be discriminated by a particular sense must belong not only to the range of the same sense, but also to the same kind of quality, e. g. colour, sound, temperature, tactile surface quality.
We find a similar picture when we come to the so called common sensibles.23 We are able to discriminate the shape of two things by the sense of sight or, alternatively (in a dark room, for example), by touch; but we could not be said to discriminate the shape of an object from its velocity by sight. That is simply not a task our eye, taken as the seat of our visual faculty, is adapted to. Thus here too identity of kind is a precondition for the discrimination of two sensible objects.
What about discriminating those sense objects which are, according to Aristotle, perceived only incidentally? The example Aristotle uses is “the son of Diares” (cf. De Anima II 6, 418a21). Although this is a relative concept, there can hardly be any doubt that the class of objects perceived incidentally comprises substances as well. Now, we can of course discriminate a man from the wall behind him, but in a case like this what we primarily discriminate is a colour patch of a particular shape from a background; the discrimination of things perceived incidentally is, as it were, parasitic to a discrimination on the level of sensible objects perceived per se, i. e. proper or common sensibles.
It is perhaps worth pointing out that we have hit here upon a central feature in Aristotle’s concept of a cognitive faculty. A cognitive faculty is defined not by its correlation to a class of objects of cognition, but by its correlation to types of difference between cognitive objects. That comes out most clearly when Aristode discusses problems with a view to introduce higher-order cognitive faculties: what comes to be called ‘the common sense’ in the Aristotelian tradition, the κύριον αἰσθητήριον in the Parva Naturalia (De Somno 2, 455a21), is first postulated as a faculty which accounts for the distinction we are able to draw between what is white and what is sweet (cf. De Anima III 2, 426a12-19; III 7, 431a20-21). Similarly, the highest cognitive faculty, νοØς (mind), is introduced because we are in a position to discriminate between a thing, like water or flesh, and its essence (cf. De Anima III 4, 429b10-13). It is a consequence of this that higher-order cognitive faculties presuppose the use of the lower ones.

V.

Let us now turn to the propositional attitudes, to the beliefs and the corresponding propositions generated by the discriminating, “critical” activity of sense perception. If perceptual activity is fundamentally an activity of discrimination, then it should be obvious that propositions of the form ‘x is different from y’ which express the belief-content brought about by an act of sense perceptual discrimination play a fundamental rôle in the framework of our perceptual knowledge. Accordingly, we should follow Aristotle and allow them a prominent place in the analysis of sense perception and perceptual knowledge, too.
It will be clear that what has been said about the relation existing between instances of ‘x’ and ‘y’ in propositions of the form ‘S discriminates x from y (by faculty f)’ directly applies to the propositions in question and to the corresponding beliefs. In particular, the relata of the relations expressed in these propositions are subject to the restriction mentioned above, a restriction to be specified according to the sense perceptual faculty used in the respective act of discrimination. Furthermore, there is a threefold classification of propositions corresponding to the three types of discrimination involving only a particular (as opposed to the common) sense that have been distinguished above. In the first class, we find propositions connecting sensible objects proper; in the second, we have connections of common sensibles. The third class, finally, embraces distinctions between things perceived incidentally.
Now, from an epistemological point of view there are important differences between those three classes of propositions. What you need in order to arrive at a (true) proposition of the first kind are standard conditions of observation. In particular, you do not have to bring in your memory or the experience gained in earlier discriminations. As soon, however, as we come to discriminating substances from each other, our memory plays an important rôle in interpreting what is perceived by us. It is not your eye, but your memory that will tell you that something in your visual field is the son of Diares.24 Similarly, if we go to the so called common sensibles: we make use of our memory and experience when we correct the foreshortenings which accompany our visual perception of shapes. And think of how we correct the illusion of the moving station when sitting on a moving train. Objects of sense perception will need interpretation by the aid of memory, if they belong to the class of common sensibles or to the class of things perceived incidentally. This is not so, if the sense objects involved are sense objects proper to a specific sense; distinctions perceived by sight, hearing, or any other sense, in this latter case, are free from interpretation presupposing the use of memory and experience.
This fact has an important consequence. For as soon as we bring in interpretations to make sense of what our five senses tell us, we are liable to deception and error. This is particularly obvious in the case of things perceived incidentally. To use Aristotle’s examples: a sense like sight or hearing is not deceived about the colour or the sound of an object, but “about what or where the coloured or sounding object is” (De Anima II 6, 418a16). But it is also clear in the case of common sensibles perceived by a particular sense. Take the perception of shapes: if they are perceived by sight, the shapes that appear to you have to be corrected according to the laws of foreshortening. In order to do so, you have to bring in what you have learned (unwittingly, for the most part) in the course of acquiring your visual skills. Thus in both cases, your experience and memory play a crucial rôle and this means that there is ample chance for deception and error to creep in. If, however, we stick to the special sensibles, we are on the safe side.
We should bear in mind that his immunity from error is not due to any special property of our sense organs but to the type of propositions involved in the propositional attitudes resulting from perception of the special sensibles: propositions not of the copulative form, but propositions expressing a difference of a very basic kind; the difference between e. g. different colours, different grades of temperature, different sounds or smells. The relata in this relation do not belong to different “categories”, to different ontological groups, or to different levels of the same group. They are, as it were, on the same ontological footing. Propositions of the copulative form are usually used to establish cross-categorial connexions or to make classifications, and even if the items brought in are objects of sense perception, the relations expressed in these propositions do not fall within the range of sense perception. And here lies the important difference with respect to those propositions used to express the difference of e. g. different colours: their difference is something that can be seen.
The picture thus emerging from Aristotle’s analysis of sense perception sheds new light on the common mistrust of much of post-Aristotelian philosophy against the senses. The stock examples used to undermine the reliability of sense perception: the oar appearing bent in water, the tower appearing round from a distance and square from nearby, the changing colours of the pigeon’s neck (cf. Sextus Empiricus, Pyrrh. Hypotyp. 1.118-120) are all cases which involve interpretation of what is actually seen and thus bring in beliefs based on memory and not merely on what is actually perceived. This will be obvious in the first two examples. But a little reflection will show that in the case of the changing colours on the pigeon’s neck we are in a similar position: for what is so upsetting in this case? It is the disappointment of our belief that colours somehow “stick” to their objects. It is a generalization about the ontological behaviour of colours that provides this case with its particular sting.
So what these examples (and similar ones) show is not that our senses ought to be mistrusted but that we should be careful in interpreting whatever information they provide. They show that we should direct our suspicion not against our senses, but against the use we make (and which we are somehow compelled to make) of our memory when applied to information provided by our senses. And in particular they do not prove (and are even quite compatible with the denial of the contention) that there cannot be a type of sense perceptual knowledge which is immune from error.

VI.

I seems to be a sound maxim of philosophical inquiry not only to detect and demolish an error but also to inquire into the reasons for its apparent plausibility – a maxim, incidentally, first formulated by Aristotle (Eth. Nic. VII 14, 1154a22-23). So I shall conclude with some observations as to why the examples just cited (and similar ones) have found so much favour with philosophers and have so persistently influenced the epistemological discussions centering around sense perception. To put this question in a somewhat altered but more specific form: Why has the rôle of propositions expressing a difference of special sensibles (colours, smells etc.) so consistently been overlooked?
The prima facie answer seems to be that the predominant position of the copulative proposition in the philosophical analysis of propositions has made people neglect other, logically less manageable forms of proposition, e. g. propositions expressing relations. But the predominance of the copulative or, for that matter, the subject-predicate proposition generally, which, at least in part, seems to be due to the impact of Aristotle’s logical interests, should perhaps not be invoked to explain a phenomenon of such a basic sort. After all, Aristotle’s Categories and the writings of his Organon generally seem to have been of rather restricted influence up to the time of Cicero. And our examples seem to come from sources not influenced by Aristotle anyway.
The proper reason for the apparent plausibility of the examples cited above will become clearer when we turn to our actual every-day talk about objects of sense perception. There we mainly use propositions of the copulative kind and we almost always cross category-boundaries in these statements. We inform each other where something is – a substance like a person or a building –, what qualities it has, how far it may be away. When we talk about relations, it is relations between things involving comparison of their qualities; we hardly ever compare two qualities as such.
The reason for this should be clear. It is information about things and their places, relations, and qualities which we need when consorting with the world. We are primarily actors in, not spectators of the world and it is our rôle as active beings which procures a prominent place for those sensibilia which come last in Aristotle’s grading i. e. those that are perceived only incidentally, namely substances. Information about things perceived incidentally almost inevitably brings in cross-categorial propositions and, hence, the possibility of error arising from the use of our memory.
This fact in itself is nothing which would be peculiarly worrying. But since these propositions, indeed, provide the basic information about the world we have to consort with, it has been inferred all along the history of epistemology after Aristotle that these propositions also ought to have a basic place in the analysis of perceptual knowledge. This inference seems to be quite unjustified, its conclusion is, as I hope to have shown, wrong. The assumption that propositions about substances should form the starting point in an analysis of sense perception may be the fundamental mistake in many philosophical theories of sense perception. Information about substances is the fruit to be plucked from the tree of sense perceptual knowledge. To gain a proper understanding of the nature of this knowledge we may be well-advised to follow Aristotle and to look for its less visible and logically rather different roots.