Laws of Thought

There are three and only three Laws of Thought. They are: the Law of Identity, the Law of Contradiction, the Law of Excluded Middle. These are the fundamental laws. They are called Universal Postulates of Thinking.

Law of Identity has been stated way back in the Bhagwadgita. What is is, what is not, is not. Whatever exists cannot be non-existent and whatever is non-existent, cannot be existent. Each thing is in identity with itself. Each thing has a fixed meaning and the fixed meaning. The meaning is definite for a particular purpose. Because it is itself, it is different from the other. 

Law of Contradiction states that A cannot both be A and Not A at the same time. Law of Contradiction is also called Law of Non Contradiction. A thinking that is valid cannot be contradictory. If we say that X is in the house, we cannot say that X is outside the house at the same time.

Law of Excluded Middle states that a thing is either A or Not A. For example, either the I perceive the bird as a parrot or not parrot. According to Jevons, the very name of the Law expresses that there is no third or middle course.