It is my view that one essential difference between persons and other creatures is to be found in the structure of a person's will. Besides wanting and choosing and being moved to do this or that, men may also want to have certain desires and motives. They are capable of wanting to be different, in their preferences and purposes, from what they are. Many animals appear to have the capacity for what I shall call "first-order desires" or "desires of the (...) first order," which are simply desires to do or not to do one thing or another. No animal other than man, however, appears to have the capacity for reflective self-evaluation that is manifested in the formation of second-order desires. (shrink)

One of Hume's most well-known works and a masterpiece of philosophy, A Treatise of Human Nature is indubitably worth taking the time to read.

John Dewey's Gifford Lectures, given at Edinburgh in 1929.