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.