These categories, then, are the fundamental, primary, or native concepts of the understanding. These flow from, or constitute the mechanism of understanding and its nature, and are inseparable from its activity. Therefore, for human thought, they are universal and necessary, or "a priori". As categories they are not contingent states or images of sensuous consciousness, and hence not to be thence derived. Similarly, they are not known to us independently of such consciousness or of sensible experience. On the one hand, they are exclusively involved in, and hence come to our knowledge exclusively through, the spontaneous activity of the understanding. This understanding is never active, however, until sensible data are furnished as material for it to act upon, and so it may truly be said that they become known to us "only on the occasion of sensible experience." For Kant, in opposition to Christian Wolff and Thomas Hobbes, the categories exist only in the mind.