He professed himself a close adherent of Hippocrates, and adopted his theory of the humours.
His general physiology was essentially founded upon the Hippocratic theory of the four elements, with which he combined the notion of spirit (pneuma) penetrating all parts, and mingled with the humours in different proportions.
The application of physiology to the explanation of diseases, and thus to practice, was chiefly by the theory of the temperaments or mixtures which Galen founded upon the Hippocratic doctrine of humours, but developed with marvellous and fatal ingenuity.
Another important point in Sydenham's doctrine is his clear recognition of many diseases as being what would be now called specific, and not due merely to an alteration in the primary qualities or humours of the older schools.
He has, indeed, described in graphic terms the greatest of the more superficial changes he underwent; how he had " carried into logical and ethical problems the maxims and postulates of physical knowledge," and had moved within the narrow lines drawn by the philosophical instructions of the class-room " interpreting human phenomena by the analogy of external nature "; how he served in willing captivity " the ` empirical ' and ` necessarian ' mode of thought," even though " shocked " by the dogmatism and acrid humours " of certain distinguished representatives "; 1 and how in a period of " second education " at Berlin, " mainly under the admirable guidance of Professor Trendelenburg," he experienced " a new intellectual birth" which " was essentially the gift of fresh conceptions, the unsealing of hidden openings of self-consciousness, with unmeasured corridors and sacred halls behind; and, once gained, was more or less available throughout the history of philosophy, and lifted the darkness from the pages of Kant and even Hegel."
