The "law of Three Stages" is familiar to many who have never read a line of his writings. That men first attempted to explain natural phenomena by the operation of imaginary deities, then sought to interpret them by abstractions, and finally came to see that they could only be understood by scientific methods, observation, and experiment--this was a generalisation which had already been thrown out by Turgot. Comte adopted it as a fundamental psychological law, which has governed every domain of mental activity and explains the whole story of human development. Each of our principal conceptions, every branch of knowledge, passes successively through these three states which he names the theological, the metaphysical, and the positive or scientific. In the first, the mind invents; in the second, it abstracts; in the third, it submits itself to positive facts; and the proof that any branch of knowledge has reached the third stage is the recognition of invariable natural laws. But, granting that this may be the key to the history of the sciences, of physics, say, or botany, how can it explain the history of man, the sequence of actual historical events? Comte replies that history has been governed by ideas; "the whole social mechanism is ultimately based on opinions." Thus man's history is essentially a history of his opinions; and these are subject to the fundamental psychological law. It must, however, be observed that all branches of knowledge are not in the same stage simultaneously. Some may have reached the metaphysical, while others are still lagging behind in the theological; some may have become scientific, while others have not passed from the metaphysical. Thus the study of physical phenomena has already reached the positive stage; but the study of social phenomena has not. The central aim of Comte, and his great achievement in his own opinion, was to raise the study of social phenomena from the second to the third stage.