This is a close scrutiny of De Anima II 5, led by two questions. First, what can be learned from so long and intricate a discussion about the neglected problem of how to read an Aristotelian chapter? Second, what can the chapter, properly read, teach us about some widely debated issues in Aristotle's theory of perception? I argue that it refutes two claims defended by Martha Nussbaum, Hilary Putnam, and Richard Sorabji: (i) that when Aristotle speaks of the perceiver becoming like the object perceived, the assimilation he has in mind is ordinary alteration of the type exemplified when fire heats the surrounding air, (ii) that this alteration stands to perceptual awareness as matter to form. Claim (i) is wrong because the assimilation that perceiving is is not ordinary alteration. Claim (ii) is wrong because the special type of alteration that perceiving is is not its underlying material realisation. Indeed, there is no mention in the text of any underlying material realisation for perceiving. The positive aim of II 5 is to introduce the distinction between first and second potentiality, each with their own type of actuality. In both cases the actuality is an alteration different from ordinary alteration. Perception exemplifies one of these new types of alteration, another is found in the acquisition of knowledge and in an embryo's first acquisition of the power of perception. The introduction of suitably refined meanings of 'alteration' allows Aristotle to explain perception and learning within the framework of his physics, which by definition is the study of things that change. He adapts his standard notion of alteration, familiar from Physics III 1-3 and De Generatione et Corruptione I, to the task of accounting for the cognitive accuracy of (proper object) perception and second potentiality knowledge: both are achievements of a natural, inborn receptivity to objective truth. Throughout the paper I pay special attention to issues of text and translation, and to Aristotle's cross-referencing, and I emphasise what the chapter does not say as well as what it does. In particular, the last section argues that the textual absence of any underlying material realisation for perceiving supports a view I have defended elsewhere, that Aristotelian perception involves no material processes, only standing material conditions. This absence is as telling as others noted earlier. Our reading must respect the spirit of the text as Aristotle wrote it.