On his return to London in 1818 he applied himself assiduously to the art of engraving, in which he acquired a skill that in after years became a most valuable assistant to his literary labours, and enabled him to illustrate his various humours and fancies by a profusion of quaint devices, which not only repeated to the eye the impressions of the text, but, by suggesting amusing analogies and contrasts, added considerably to the sense and effect of the work.
Galen believed in the doctrine of humours originated by Hippocrates, which supposes the condition of the body to depend upon the proper mixture of the four elements, hot, cold, moist and dry, and that drugs possess the same elementary qualities, and that on the principle of contraries one or other was indicated, e.g.
The power of Naevius was the more genuine Italian gift - the power of satiric criticism - which was employed in making men ridiculous, not, like that of Plautus, in extracting amusement from the humours, follies and eccentricities of life.
According to this celebrated theory, the body contains four humours - blood, phlegm, yellow bile and black bile, a right proportion and mixture of which constitute health; improper proportions or irregular distribution, disease.
But he recognized, at least in acute diseases, a natural process which the humours went through - being first of all crude, then passing through coction or digestion, and finally being expelled by resolution or crisis through one of the natural channels of the body.
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.
One sentence of Locke's, in a letter to William Molyneux, sums up the practical side of Sydenham's teaching: "You cannot imagine how far a little observation carefully made by a man not tied up to the four humours [Galen], or sal, sulphur and mercury [Paracelsus], or to acid and alcali [Sylvius and Willis] which has of late prevailed, will carry a man in the curing of diseases though very stubborn and dangerous; and that with very little and common things, and almost no medicine at all."
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."
He turned the eyes of his contemporaries from the commonplace social humours of later Greek life to the contemplation of the heroic age.
The humours and passions and diseases of different nations are different, and the physician must go among the nations if he will be master of his art; the more he knows of other nations, the better he will understand his own.
He argued that the different humours of the human eye so refract rays of light as to produce an image on the retina which is free from colour, and he reasonably argued that it might be possible to produce a like result by combining lenses composed of different refracting media.'
Like Gregory and Hall, he argued that, since the various humours of the human eye were so combined as to produce a perfect image, it should be possible by suitable combinations of lenses of different refracting media to construct a perfect object-glass.
Depicting feudalism in the vivid colours of an age at war with feudal institutions, breathing into antique histories the breath of actual life, embracing the romance of Italy and Spain, the mysteries of German legend, the fictions of poetic fancy and the facts of daily life, humours of the moment and abstractions of philosophical speculation, in one homogeneous amalgam instinct with intense vitality, this extraordinary birth of time, with Shakespeare for the master of all ages, left a monument of the Re- naissance unrivalled for pure creative power by any other product of that epoch.