 First, however, it is important to get more precise about how exactly functional definition is supposed to work. This can be done by focusing on a general method for constructing functional definitions introduced by David Lewis (1972; building on an idea of Frank Ramsey's), which has become standard practice for functionalists of all varieties. Articulating this method will help in evaluating the strengths and weaknesses of the different varieties of functionalism—while displaying some further challenges that arise for them all.