Wydawn. Nauk. Uniwersytetu Mikołaja Kopernika (2009
Each of us is a measure. The project of advocates
of change in Plato’s Theaetetus as compared with
One of the most intriguing motives in Plato’s Theaetetus is its historical-based division of philosophy, which revolves around the concepts of rest (represented by Parmenides
and his disciples) and change (represented by Protagoras, Homer, Empedocles, and
Epicharmus). This unique approach gives an opportunity to reconstruct the views of
marginalized trend of early Greek philosophy - so called „the sophistic movement”.
Paradoxically, previous research shows little interest in sophistic thought as
a source of the standpoint of advocates of change („the secret doctrine”). The roots
of „the secret doctrine” were investigated in the works of Heraclitus, Aristippus, and
Antisthenes or those related to “neoheracliteanism”. However, researchers did not make
any significant attempt to confront this concept with the contemporary research on
the sophistic movement. The conviction that sophistry was primarily humanistically oriented
was one of the main reasons why researches were opposed to the fact that „the
secret doctrine” could represent a true expression of Protagoras’ views. This is why
J. Burnet and F. M. Cornford in their seminal works assumed that “the secret doctrine”
should be attributed to Plato, who simply combined a series of loose statements into one
In this work, we argue that the thesis which questions the parallels between the
sophists’ interests and the philosophers of nature requires a significant revision. There is
ample evidence to suggest that the philosophy of nature was a part of sophists’ research.
This is supported by two main arguments. First, the tutors of sophists were philosophers
of nature. Second, there are numerous sources that explicitly show sophists’ interest in
the physical issues. These sources include anecdotal evidence about the fact that sophists
wrote works On nature. There is also information confirming that they deliberated on
detailed physical issues.
The analogies between the concepts attributed to the advocates of change and our
knowledge about sophists from other sources is very wide and contains most elements,
which are included in the project of “changeable reality” presented in Theaetetus. The
deliberations on the mechanism of perception, which are close to those of flux theory
of perception in Theaetetus, are present in the sources referring to Gorgias of Leontinoi,
the famous sophist and rhetorician. Also, the second element of “the secret doctrine”
that is the metaphysics of flux matches up with what we know about the sophists’ views
from other sources. On this basis, one can deduce that – contrary to the tradition which
marginalized the role of sophistic considerations on the issue of being and non-being – it
was one of the major subjects of sophistic research. Its main point was the criticism of
the Eleatic conception of a single and unchangeable being, which also plays a key role
in the doctrine of flux in Theaetetus.
The epistemological theses which are presented in Theaetetus are borne out in
sophistic sources. They include the definition of knowledge as perception, the „Man-measure”
formula and a number of principles, which result from these foundational
theses. Sophists’ empirical preferences resonate with the theses of the advocates of
change in Theaetetus. Special attention that is given to the issues of differences among
people, and even to cognitive differences in one person depending on the changeable
states to which a person is subject, goes well together with what we know about
reciprocal influence between the sophistry and medicine. The consequences of the
epistemological conception present in Theaetetus have their equivalents in sophists’
works and other testimonies. An example of these consequences may be the abolition
of truth and falsehood or the abolition of contradiction, which finds its expression in the
thesis ouk estin antilegein.
The analogies also concern reflections on the language itself. The project of the
“new language” uses categories, which were developed by sophists. These include the
antithesis of nomos and physis. The general intentions of this project reflect Protagoras’
ideas, at least to the extent to which they are known from the sources reporting his
thoughts on language.
Plato’s Theaetetus can thus be considered a veritable treasury of sophistic motifs.
Even though the problem remains unsolved and one is still not able to unambiguously
decide about the author of “the secret doctrine”, one can come to a certain conclusion –
even if Plato synthesized various doctrines, he must have relied in his project mostly on
the elements that he borrowed from sophists.
Moreover, the value of reconstructing the project of the advocates of change in
Theaetetus does not consist of mere enumeration of sophistic motifs. The dialogue is
key to understanding the sophistic movement, whose separate doctrines – for the lack
of sources and as a result of centuries-old disregard – are usually treated as rhetorical
formulae that are interpreted in many ways and have no philosophical foundations.
If it is really the case that the theses attributed to Protagoras in Theaetetus were actually
a part or a derivative of Protagoras’ thought, or – speaking more conservatively – if
they constitute a synthesis of sophistic thought done by Plato, they could represent
philosophical foundations for the most important sophistic theses: the “Man-measure”
formula, the ouk estin antilegein principle, the concept of language as a tool, the idea
of the relativity of good and the whole practical sphere of sophists’ activity. Contrary to
the views of many researchers, we are certain that the representatives of the sophistic
movement did not limit themselves only to the application of practical rules, which
determined the extent of their educational or rhetorical-political activity. They were
capable of creating – indeed, they did create comprehensive projects that embraced the
whole thematic scope subject to philosophical reflection.