A reasonable choice between Skinner's and Chomsky's theories requires reference to a conception of human nature. It is explained in detail why this is so, in the context of an analysis of what it is to ‘choose’ a theory. This account helps to explain the unity and coherence of the science, methodology, conception of science, object of scientific inquiry and views towards control of each of Skinner and Chomsky, and thereby explains the chasm which separates the parties to their respective programs. The analysis given implies that, in a precise sense, the theory-choice is implicated in value-judgments.