With respect to the former point, Spinoza's masterly examination of the Passions in the third part of the _Ethics_ should have been known to Hume.[20] But, if he had been acquainted with that wonderful piece of psychological anatomy, he would have learned that the emotions and passions are all complex states, arising from the close association of ideas of pleasure or pain with other ideas; and, indeed, without going to Spinoza, his own acute discussion of the passions leads to the same result,[21] and is wholly inconsistent with his classification of those mental states among the primary uncompounded materials of consciousness. If Hume's "impressions of reflection" are excluded from among the primary elements of consciousness, nothing is left but the impressions afforded by the five senses, with pleasure and pain. Putting aside the muscular sense, which had not come into view in Hume's time, the questions arise whether these are all the simple undecomposable materials of thought? or whether others exist of which Hume takes no cognizance. Kant answered the latter question in the affirmative, in the _Kritik der reinen Vernunft_, and thereby made one of the greatest advances ever effected in philosophy; though it must be confessed that the German philosopher's exposition of his views is so perplexed in style, so burdened with the weight of a cumbrous and uncouth scholasticism, that it is easy to confound the unessential parts of his system with those which are of profound importance. His baggage train is bigger than his army, and the student who attacks him is too often led to suspect he has won a position when he has only captured a mob of useless camp-followers. In his _Principles of Psychology_, Mr. Herbert Spencer appears to me to have brought out the essential truth which underlies Kant's doctrine in a far clearer manner than any one else; but, for the purpose of the present summary view of Hume's philosophy, it must suffice if I state the matter in my own way, giving the broad outlines, without entering into the details of a large and difficult discussion. When a red light flashes across the field of vision, there arises in the mind an "impression of sensation"--which we call red. It appears to me that this sensation, red, is a something which may exist altogether independently of any other impression, or idea, as an individual existence. It is perfectly conceivable that a sentient being should have no sense but vision, and that he should have spent his existence in absolute darkness, with the exception of one solitary flash of red light. That momentary illumination would suffice to give him the impression under consideration; and the whole content of his consciousness might be that impression; and, if he were endowed with memory, its idea.