"Thinking" is an essentially human activity occurring in two basic forms. We may think in order to attain knowledge of what is, must, or may be the case; we also may think with a view to making up our mind about what we will or will not do. Following Aristotle, these two forms of thought may be called, respectively, contemplation and deliberation. Both forms may be carried on well or badly, successfully or unsuccessfully, intelligently or stupidly. When contemplation is successful, it terminates in a conclusion; successful deliberation terminates in a decision or resolution. Again following Aristotle, the form of reasoning involved in contemplation may be called theoretical, and the form involved in deliberation may be called practical. Obviously, our day-by-day reasoning in ordinary life is an untidy mixture of both these basic forms.

Less generally, thinking is commonly understood as a largely covert activity, something done mainly...