In this paper I introduce and critically examine a paradox about perceiving that is in some ways analogous to the paradox about meaning which Kripke puts forward in his exegesis of Wittgenstein's views on Rule-following. When applied to vision, the paradox of perceiving raises a metaphysical scepticism about which object a person is seeing if he looks, for example, at an apple on a tree directly in front of him. Physical objects can be seen when their appearance is distorted in various ways by illusions. The question therefore arises as to how can we answer the sceptic who suggests the following: although the viewer appears to be seeing the green apple in front of him, he is actually suffering a bizarre illusion of a blue car situated somewhere behind him. The sceptic is not concerned with epistemic problems about how we know which object, if any, the subject is seeing; the sceptic is raising the more fundamental question: what fact of the matter underlies a person's perceptual relation to the physical world, in virtue of which that person may be justified in arriving at a perceptual belief about the environment? Among the various different issues raised by the sceptic, I focus on the question: what determines the perceiving relation? I canvass a number of possible proposals in answer to it, concentrating mainly on two opposed accounts: the Disjunctive View and the Causal Theory of Perception. I argue in particular for the following two claims: that the paradox highlights the fact that the Disjunctive View fails to provide a coherent positive account of what perceiving is. that the problem of 'deviant causal chains', often thought to raise particular difficulties for the Causal theorist, can also be raised against other accounts of perception, including versions of the Disjunctive View. I conclude that unless the Causal Theory of Perception can be upheld, there will be no way of answering the sceptic.