The Way of Ideas theorists argued that only if we concede that “ideas” are the sole immediate objects of apprehension can we explain how a sequence of physical and neurological events could cause perception. Reid's polemic against this argument, as I have presented it thus far, came in two parts. First he argued that perception is not in fact explained by hypothesizing ideas. And second, he objected to the principles that the Way of Ideas theorists accepted for a satisfactory explanation. The crucial principle was the one I have been calling “the identity principle”: The immediate object of an act of apprehension is identical with the immediate cause thereof. Reid's response was that there is no good reason whatsoever to accept this principle; quite to the contrary, there are good reasons for rejecting it.

Reid has a bit more to say about the failure of the Way of Ideas theory to explain what it set out to explain; it also fails to explain the belief component of perception. But before we get to that, let's look at another aspect of Reid's argument against the claims of the Way of Ideas theorists concerning the conception (i.e., apprehension) which is ingredient in perception. Specifically, let's look at what he says concerning the claim of the Way of Ideas theorists that in perception the immediate object of apprehension is a sense datum that represents the external object perceived, and that the way to think of these mental representations is on the model of reflective images.