Lucy Allais argues that we can better understand Kant's transcendental idealism by taking seriously the analogy of appearances to secondary qualities that Kant offers in the Prolegomena. A proper appreciation of this analogy, Allais claims, yields a reading of transcendental idealism according to which all properties that can appear to us in experience are mind-dependent relational properties that inhere in mind-independent objects. In section 1 of my paper, I articulate Allais's position and its benefits, not least of which is its elegant explanation of how the features of objects that appear to us are transcendentally ideal while still being ‘empirically’ real. In section 2, I contend that there are elements of Allais's account that are problematic, yet also inessential, to what I view to be the core contribution of her analysis. These elements are the views that the properties that appear to human beings are not really distinct from properties that things have ‘in themselves’ and that Kant embraced a relational account of perception. In section 3, I return to the core of Allais's reading and argue that, despite its multiple virtues, it cannot make sense of key features of Kant's idealism.