Comte begins with the natural sciences and postulates the doctrine of evolution. To the definition of this doctrine he makes some interesting approaches. The discussion of the order and arrangement of the various sciences and of their characteristic differences is wonderful in its insight and suggestiveness. He asserts that in the study of nature we are concerned solely with the facts before us and the relations which connect those facts. We have nothing to do with the supposed essence or hidden nature and meaning of those facts. Facts and the invariable laws which govern them are the only legitimate objects of pursuit. Comte infers that because we can know, in this sense, only phenomena and their relations, we should in consequence guard against illusions which creep in again if we so much as use the words principle, or cause, or will, or force. By phenomena must be understood objects of perception, to the exclusion, for example, of psychological changes reputed to be known in self-consciousness. That there is no knowledge but of the physical, that there is no knowing except by perception--this is ever reiterated as self-evident. Even psychology, resting as it does largely upon the observation of the self by the self, must be illusive. Physiology, or even phrenology, with the value of which Comte was much impressed, must take its place. Every object of knowledge is other than the knowing subject. Whatever else the mind knows, it can never know itself. By invincible necessity the human mind can observe all phenomena except its own. Commenting upon this, James Martineau observed: 'We have had in the history of thought numerous forms of idealism which construed all outward phenomena as mere appearances within the mind. We have hitherto had no strictly corresponding materialism, which claimed certainty for the outer world precisely because it was foreign to ourselves.' Man is the highest product of nature, the highest stage of nature's most mature and complex form. Man as individual is nothing more. Physiology gives us not merely his external constitution and one set of relations. It is the whole science of man. There is no study of mind in which its actions and states can be contemplated apart from the physical basis in conjunction with which mind exists.