During the busy years of my professional career, I had snatched leisure for some professional treatises, which had made more or less sensation, and one of them, entitled "The Vital Principle; its Waste and Supply," had gained a wide circulation among the general public. This last treatise contained the results of certain experiments, then new in chemistry, which were adduced in support of a theory I entertained as to the re-invigoration of the human system by principles similar to those which Liebig has applied to the replenishment of an exhausted soil,--namely, the giving back to the frame those essentials to its nutrition, which it has lost by the action or accident of time; or supplying that special pabulum or energy in which the individual organism is constitutionally deficient; and neutralizing or counterbalancing that in which it super-abounds,--a theory upon which some eminent physicians have more recently improved with signal success. But on these essays, slight and suggestive, rather than dogmatic, I set no value. I had been for the last two years engaged on a work of much wider range, endeared to me by a far bolder ambition,--a work upon which I fondly hoped to found an enduring reputation as a severe and original physiologist. It was an Inquiry into Organic Life, similar in comprehensiveness of survey to that by which the illustrious Muller, of Berlin, has enriched the science of our age; however inferior, alas! to that august combination of thought and learning in the judgment which checks presumption, and the genius which adorns speculation. But at that day I was carried away by the ardour of composition, and I admired my performance because I loved my labour. This work had been entirely laid aside for the last agitated month; now that Lilian was gone, I resumed it earnestly, as the sole occupation that had power and charm enough to rouse me from the aching sense of void and loss. The very night of the day she went, I reopened my manuscript. I had left off at the commencement of a chapter Upon Knowledge as derived from our Senses. As my convictions on this head were founded on the well-known arguments of Locke and Condillac against innate ideas, and on the reasonings by which Hume has resolved the combination of sensations into a general idea to an impulse arising merely out of habit, so I set myself to oppose, as a dangerous concession to the sentimentalities or mysticism of a pseudo-philosophy, the doctrine favoured by most of our recent physiologists, and of which some of the most eminent of German metaphysicians have accepted the substance, though refining into a subtlety its positive form,--I mean the doctrine which Muller himself has expressed in these words:--