 If my maxim passes the universal law test, then it is morally permissible for me to act on it, but I fully exercise my autonomy only if my fundamental reason for acting on this maxim is that it is morally permissible or required that I do so. Imagine that I am moved by a feeling of sympathy to formulate the maxim to help someone in need. In this case, my original reason for formulating this maxim is that a certain feeling moved me. Such feelings are not entirely within my control and may not be present when someone actually needs my help. But this maxim passes Kant’s test: it could be willed as a universal law that everyone help others in need from motives of sympathy. So it would not be wrong to act on this maxim when the feeling of sympathy so moves me.  But helping others in need would not fully exercise my autonomy unless my fundamental reason for doing so is not that I have some feeling or desire, but rather that it would be right or at least permissible to do so. Only when such a purely formal principle supplies the fundamental motive for my action do I act autonomously.