The term "laws of thought" traditionally covered the principles of identity, of contradiction, of excluded middle, and occasionally the principle of sufficient reason. Whereas these principles were frequently discussed from the time of the Greeks until the beginning of the twentieth century, the term has become obsolete, for at least two good reasons. One is the great and confusing variety of meanings with which it has been used, the other is the now generally acknowledged fact that no viable system of logic can be constructed in which the principles of identity, contradiction, and excluded middle would be the only axioms. Typical discussions of these principles are to be found, for example, in Friedrich Ueberweg's System der Logik and in H. W. B. Joseph's Introduction to Logic. In the following discussion the principle of sufficient reason, which, unlike the others, cannot be interpreted as a...