 In its strictest sense the term ‘practical reason’ (ratio practica) refers specifically to a type of reasoning, which is analogous to the deductions of speculative or theoretical science. When discussing human knowledge of the precepts of natural law Thomas Aquinas argues that these commands are related to practical reasoning as the first principles of demonstration are related to speculative reasoning; in each science there are certain principles of demonstration which are known in themselves (principia per se nota). These principles, which comprise the universal laws of moral behavior direct all subsequent moral reasoning. Thomas bases his theory of correct reasoning on the human ability to discover an underlying order in any field of inquiry. The order of reasoning which determines metaphysical knowledge is derived from a recognition of the principle of non-contradiction. Reasoning from the notion that being and non-being are contradictory terms, a metaphysician argues for certain conclusions about the nature of being. In practical reason one begins with the principle that all human acts are directed to an end and then comes to recognize the fundamental element of ethics to be ‘do good and avoid evil’: