Kant sees the moral law as a categorical imperative not because we are rational beings but because we are sensible, biological beings who need practical reason to restrain natural desire. On the other hand, since it is human nature to pursue happiness and satisfy natural desires, it is also our duty to look after our own well-being. Practical reason does not demand from us that we cast happiness aside. The resolution of this contradiction is the gist of the Dialectic of Pure Practical Reason.