Page:Philosophical Review Volume 3.djvu/692

676 In both works Hume unhesitatingly affirms the possibility of science, but in the Inquiry he does not shake so severely as in the Treatise the certainty of science. This change, however, is effected more by omissions than by modifications. The difference of treatment in the later work, of these two questions, seems to be due, partly at least, to the different subject-matter dealt with in the respective introductions,—the introduction to the Treatise of Human Nature being primarily an introduction to the science of man, and the first section of the Inquiry being primarily an introduction to the theoretical part of the science of man. But other circumstances may have contributed a share to the common result, for instance, Hume's desire for fame as a philosopher, and his change of temperament with increased experience that led him often to regret the positive air that pervaded the Treatise.

2. Perceptions, their nature and cause. Impressions and ideas are the only objects of human knowledge. And they all, although "conjoined" in a sense, are not "connected," but are distinct and separable existences. On these points both works wholly agree. Concerning the cause of perceptions, Hume evidently gives a twofold account, that of the psychological cause and that of the physiological, (1) Psychologically, impressions of sense are original elements or facts, and ideas of sense are derived from them. Impressions of reflection are the cause of ideas of reflection, and are themselves the effect, or sometimes apparently the attendant, of impressions or ideas of sense. (2) The physiological cause of perceptions is motion in the brain or nerves, or of the animal spirits. Thus far in regard to the immediate cause of perceptions. But now if we ask the further question, What is the ultimate cause of perceptions? Hume has no answer to give. It is both unknown and unknowable. On all of these points respecting impressions