that-- "What we call a mind is nothing but a heap or collection of different perceptions, united together by certain relations, and supposed, though falsely, to be endowed with a perfect simplicity and identity."--(I. p. 268.) With this "nothing but," however, he obviously falls into the primal and perennial error of philosophical speculators--dogmatising from negative arguments. He may be right or wrong; but the most he, or anybody else, can prove in favour of his conclusion is, that we know nothing more of the mind than that it is a series of perceptions. Whether there is something in the mind that lies beyond the reach of observation; or whether perceptions themselves are the products of something which can be observed and which is not mind; are questions which can in nowise be settled by direct observation. Elsewhere, the objectionable hypothetical element of the definition of mind is less prominent:-- "The true idea of the human mind is to consider it as a system of different perceptions, or different existences, which are linked together by the relation of cause and effect, and mutually produce, destroy, influence and modify each other.... In this respect I cannot compare the soul more properly to anything than a republic or commonwealth, in which the several members are united by the reciprocal ties of government and subordination, and give rise to other persons who propagate the same republic in the incessant changes of its parts."--(I. p. 331). But, leaving the question of the proper definition of mind open for the present, it is further a matter of direct observation, that, when we take a general survey of all our perceptions or states of consciousness, they naturally fall into sundry groups or classes. Of these classes, two are distinguished by Hume as of primary importance. All "perceptions," he says, are either "_Impressions_" or "_Ideas_." Under "impressions" he includes "all our more lively perceptions, when we hear, see, feel, love, or will;" in other words, "all our sensations, passions, and emotions, as they make their first appearance in the soul" (I. p. 15). "Ideas," on the other hand, are the faint images of impressions in thinking and reasoning, or of antecedent ideas. Both impressions and ideas may be either _simple_, when they are incapable of further analysis, or _complex_, when they may be resolved into simpler constituents. All simple ideas are exact copies of impressions; but, in complex ideas, the arrangement of simple constituents may be different from that of the impressions of which those simple ideas are copies.