The final objection to the identity theory of secondary qualities, the resemblance objection, merits attention on historical grounds. The resemblance objection depends on two commitments for which today's philosophers exhibit relatively little sympathy. The first is the atomic or corpuscular theory of matter, which in the seventeenth and eighteenth centuries was rapidly replacing its Scholastic predecessors. The second is the view that immediate objects of perception are mental. That is, the resemblance objection assumes the falsity of Direct Realism and the truth of either Indirect Realism or Idealism. In the context of this discussion, such a starting point is question begging. But the resemblance objection represents the most prominent argument for the mentality of secondary qualities from Reid's day and one with which he is deeply concerned, because of its conceptual links to the Way of Ideas. The following pages attempt to reconstruct the resemblance objection in accordance with Reid's analysis of the Way of Ideas and to highlight the features of his doctrine of primary and secondary qualities that bear on his response to that tradition.

According to Reid, adherents of the Way of Ideas include Platonists, Aristotelians, Epicureans, Cartesians and Lockeans, the only noted exception being Antony Arnauld. Reid holds the Way of Ideas responsible for the external world scepticism of Berkeley and Hume. Among Scholastics, the resemblance theory manifests itself in terms of physical causation by contagion of forms. Just as they explain heat transfer by saying that one warm body produces a form of warmness in another, so in perception a red object transmits a ‘sensible form’ of redness to the eyes and mind. The phenomenological experience is of the same form as the physical quality. So generations of philosophers rely on the Way of Ideas to account for the intentionality and epistemic accuracy of perceptions or, more accurately, of sensations. The sensible form impressed on the mind resembles the object of perception in virtue of sharing its formal cause. And this resemblance makes the resulting idea or sensible form both literally informative and of the object.

According to Reid, early moderns take the immediate objects of all thought to be mental entities, ideas or images, following the Scholastic model of sensible forms.