 Kant developed the other variant. He took Hume’s ‘tendencies of the mind to pass’ from one idea to another, without which we could not construct the world, and canonised them as a priori categories of the understanding. Hume’s empiricist emphasis is psychological. It concerns what we are habituated to do. Because of his empiricism, he will not bring non-psychological necessity into it. Nevertheless, there is the implication that making these transitions is the only way in which one can understand the world. Kant drops the empirical psychology and makes it a matter of a priori psychology, that only by employing certain categories could we have experience as of a physical world. It is only by understanding the world as possessing enduring spatio-temporal objects, which enter into causal relations with each other (that is, it is only by applying the categories of substance and causation) that we can have intelligible experience. Substances—that is, a framework of stable, enduring objects—are essential, but the source of this necessity lies not in how the world is in itself, but in the framework that we are obliged to impose.