Page:Russell - The Problems of Philosophy, 1912.djvu/117

Rh these others, so long as they are simple, are just as obvious as the principles taken for granted. For no very good reason, three of these principles have been singled out by tradition under the name of "Laws of Thought."

They are as follows:

(1) The law of identity: "Whatever is, is."

(2) The law of contradiction: "Nothing can both be and not be."

(3) The law of excluded middle: "Everything must either be or not be."

These three laws are samples of self-evident logical principles, but are not really more fundamental or more self-evident than various other similar principles: for instance, the one we considered just now, which states that what follows from a true premiss is true. The name "laws of thought" is also misleading, for what is important is not the fact that we think in accordance with these laws, but the fact that things behave in accordance with them; in other words, the fact that when we think in accordance with them we think