Abstract

When we look at a tree, two images of it are formed, one on each of our retinas. Why, then, asks the child or the philosopher, do we not see two trees?1 Thomas Reid offers an answer to this question in the section of his Inquiry into the Human Mind entitled ‘Of seeing objects single with two eyes’. The principles he invokes in his answer serve at the same time to explain why we do occasionally see objects double. In Part I of this essay, I examine the principles Reid uses to explain single and double vision. This part is mostly an exercise in the history of cognitive science, but it raises questions of interest to philosophers along the way. In Part II, I turn to a hard-core philosophical problem raised by double vision, namely, whether double vision constitutes an objection to the direct realist theory of perception, which was one of Reid's main philosophical purposes to promote