The distinction at the heart of Kant's philosophy is a metaphysical distinction: things in themselves are substances, bearers of intrinsic properties; phenomena are relational properties of substances. Kant says that things as we know them are composed "entirely of relations", by which he means forces. Kant's claim that we have no knowledge of things in themselves is not idealism, but humility: we have no knowledge of the intrinsic properties of substances. Kant has an empiricist starting-point. Human beings are receptive creatures. (...) We must be affected by the things of which we come to have knowledge. Kant believes that humility follows from this fact of receptivity. Humility does follow from receptivity, once a further premise is supplied. Kant believes that relational properties, causal powers in particular, are not reducible to intrinsic properties, and that intrinsic properties are therefore causally inert. Receptivity implies that we can have knowledge only of what can affect us. Irreducibility implies, in Kant's view, that intrinsic properties cannot affect us. Humility follows from these. This interpretation finds support in a range of critical and pre-critical writings, but there is a special focus on an early anti-Leibnizian argument for irreducibility that has considerable philosophical merit. One advantage of this interpretation is that the following famous contradiction is dissolved: things in themselves exist, and are the causes of phenomena, and we have no knowledge of them as they are in themselves. Moreover Kant's scientific realism, surprising on an idealist interpretation, makes sense. It does not conflict with Kant's claim that he makes all the qualities secondary--he means that he makes them powers, or in Locke's terms, tertiary qualities. Kant's "primary"/secondary quality distinction is superior to widely accepted alternatives, and his application of the irreducibility argument to the primary qualities challenges some current orthodoxies. Kant's commitment to the unobservables of science is permitted by his understanding of receptivity: we can have knowledge of anything that can affect us. Kant's scientific realism, and his humility, thus have a common source. (shrink)