ABSTRACT
All logical thinking exemplifies certain abstract principles in accordance with which such thinking takes place. It is extremely difficult to determine what these principles are, and which, if any, are independent of the rest. Such an interpretation is clearly metaphysical; it expresses a theory with regard to the nature of persistent individuality. This could not properly be regarded as a fundamental principle of logical thinking, so that this interpretation need not be discussed here. Aristotle did not himself formulate any "law of identity", but such a law might be extracted from his assertion, 'Everything that is true must in every respect agree with itself'. Fundamental logical principles cannot be proved in any absolute sense, for all proof must presuppose them. To think logically some principles must be assumed, since logical thinking is thinking in accordance with logical principles. Logicians have been wont to raise the question whether logic is a science or an art.
