What is the Difference Between a Principle and an Element?

The followers of Aristotle and Plato conclude that elements are discriminated from principles. Thales the Milesian supposeth that a principle and the elements are one and the same thing, but it is evident that they vastly differ one from another. For the elements are things compounded; but we do pronounce that principles admit not of a composition, nor are the effects of any other being. Those which we call elements are earth, water, air, and fire. But we call those principles which have nothing prior to them out of which they are produced; for otherwise not these themselves, but rather those things whereof they are produced, would be the principles. Now there are some things which have a pre-existence to earth and water, from which they are begotten; to wit, matter, which is without form or shape; then form, which we call [Greek omitted] (actuality); and lastly, privation. Thales therefore is most in error, by affirming that water is both an element and a principle.