In this paper I argue that Wittgenstein's aim in the aspect-perception passages is to critically evaluate a specific hypothesis. The target hypothesis in these passages is the Gestalt psychologist Köhler's “isomorphism principle.” According to this principle, there are neural correlates of conscious perceptual experience, and these neural correlates determine the content of our perceptual experiences. Wittgenstein's argument against the isomorphism principle comprises two steps. First, he diffuses the substantiveness of the principle by undermining an important assumption that underpins this principle, namely, that there is a unitary concept of seeing. Next, Wittgenstein argues that some forms of aspect-perception involve recognitional capacities, the exercise of which is normatively constrained. The normative nature of aspect-perceiving plays a pivotal role in Wittgenstein's rejection of the isomorphism principle. Aside from the clear exegetical benefits gained from identifying the target hypothesis in the aspect-perception passages as the isomorphism principle, construing the remarks in the way suggested here is also philosophically interesting in its own right: it shows Wittgenstein engaging directly in the mind–body problem, construed as the problem of intentionality.