The world for Fichte is at bottom a spiritual order, the revelation of a self-determining ego or reason; hence the science of the ego, or reason, the _Wissenschaftslehre_, is the key to all knowledge, and we can understand nature and man only when we have caught the secret of the self-active ego. Philosophy must, therefore, be _Wissenschaftslehre_, for in it all natural and mental sciences find their ultimate roots; they can yield genuine knowledge only when and in so far as they are based on the principles of the Science of Knowledge--mere empirical sciences having no real cognitive value. The ego-principle itself, however, without which there could be no knowledge, cannot be grasped by the ordinary discursive understanding with its spatial, temporal, and causal categories. Kant is right: if we were limited to the scientific intellect, we could never rise above the conception of a phenomenal order absolutely ruled by the causal law. But there is another source of knowledge: in an act of inner vision or intellectual intuition, which is itself an act of freedom, we become conscious of the universal moral purpose; the law of duty or the categorical imperative commands us to be free persons. We cannot refuse to accept this law without abandoning ourselves as persons, without conceiving ourselves as _things_, or mere products of nature; the choice of one's philosophy, therefore, depends upon what kind of man one is--upon one's values, upon one's will. The type of man who is a slave of things, who cannot raise himself out of the causal mechanism, who is not free, will never be able to conceive himself otherwise than as a cog in a wheel. Fichte accepts the ego, or spirit, as the ultimate and absolute principle, because it alone can give our life worth and meaning. Thus he grounds his entire philosophy upon a moral imperative which presents itself to the ego in an inner vision. He also tells us that we can become immediately aware of the pure activity of the ego, of our free action, in a similar act of intellectual intuition. But we cannot know this free act unless we perform it ourselves; no one can understand the idealistic philosophy who is not free; hence philosophy begins with an act of freedom--_im Anfang war die Tat_.