What does Aquinas mean when he speaks of ‘spiritual’ change in explaining sense perception? This chapter is partly exegesis of that notion, partly explication of the author’s own previous invocation of ‘spiritual’ change to characterise the way perception for Aristotle takes on sensible form without matter, and partly explanation of why in philosophy since Descartes it is so difficult to understand how for Aristotle and Aquinas perception is both physical and mental. For both thinkers, it is an ‘unordinary’ physical alteration. That does not make it a ‘mental’ event in any sense that contrasts with ‘physical’. In sight the eye is coloured by the sensible form of red conceived not as a natural form, but as what Aquinas calls an intentio. The notion of intentio here expresses the idea of cognitive awareness, or of sensible form causing knowledge in a being which has the power of cognition. Aquinas calls sight the ‘most spiritual’ of the senses because, whereas with the operation of the other senses ordinary natural processes are required either as causal antecedents or concomitant effects, in sight there is no similar natural change at all. In its power of cognition alone we reach the end of explanation.