 If we construe Reid as holding that in the act of perception there are three elements—the physiological modification of the organ, a mental sensation and the perception of an object—then we can contrast this with Hamilton's position which holds that the mental sensation and the perception are simultaneous and in a sense two sides of the same coin. Reid holds, of course, that we do not reason from sensation to perception; the apprehending mind moves from one to the other by a natural, inbuilt instinct—one of the principles of common sense. Hamilton too holds that there is no reasoning process here, but he also thinks that the continuing division that Reid is employing between sensation and perception is incompatible with the idea of immediate perception or direct realism. Hence his amendment, which so to speak ties the sensation and perception together. But how is this further contention to be sustained? Is it a conceptual truth of some kind, or an empirical observation about how the mind works? Hamilton's writings in general tend to assertion more than argument, and while he has a great deal to say on this point, it does seem that his “solution” to the problem of perception is an arbitrary stipulation designed to overcome it. At any rate, if we do press the question of its defence, we quickly encounter a new version of the old division, namely whether the perception is to be identified as a manifestation of self-evident principles of common sense, or as a psychological association of ideas. In this sense Hamilton's thesis is still set within the fundamental parameters of the Hume/Reid debate. 