In a brief but remarkable section of the Inquiry into the Human Mind, Thomas Reid argued that the visual field is governed by principles other than the familiar theorems of Euclid—theorems we would nowadays classify as Riemannian. On the strength of this section, he has been credited by Norman Daniels, R. B. Angell, and others with discovering non-Euclidean geometry over half a century before the mathematicians—sixty years before Lobachevsky and ninety years before Riemann. I believe that Reid does indeed have (...) a fair claim to have discovered a non-Euclidean geometry. However, the real basis of Reid’s geometry of visibles is subtle and easy to misidentify. My main aim in what follows is to make clear what the real basis is, separating it from several fallacious or irrelevant considerations on which Reid may seem to be relying. A secondary aim is to air the worry that Reid’s case for his geometry can succeed only at the cost of compromising the achievement for which Reid is best known—his direct realist theory of perception. (shrink)

Thomas Reid thought of himself as a critic of the representative theory of perception, of what he called the ‘theory of ideas’ or ‘the ideal theory’.2 He had no kind words for that theory: “The theory of ideas, like the Trojan horse, had a specious appearance both of innocence and beauty; but if those philosophers had known that it carried in its belly death and destruction to all science and common sense, they would not have broken down their walls to (...) give it admittance.”3 Many have supposed that his opposition to the representative theory was grounded in his direct realism.4 A direct realist theory of perception holds that perception of external objects is not mediated by any mental entity whose intrinsic character licenses a move from the mental entity to the external object presented in perception. Reid himself, in an oration of 1759, delivered at graduation ceremonies over which he presided as regent and professor of philosophy at King’s College in Aberdeen, said that he did not “understand what need there is of an intermediate object for thought about something to be possible.”5 Hence, if Reid was not a direct realist, philosophers and historians would have to ask whether and to what degree Reid was what he thought himself to be. (shrink)