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When Hume examined, with more and more minuteness, the elements of the materials on which the mind works, he could still find nothing but these impressions and ideas. Looking at language as a machinery for giving expression to thought, he thus established for himself a test of its adaptation to its right use,—a test for discovering whether in any given case it really served the purpose of language, or was a mere unmeaning sound. As he found that there was nothing on which thought could operate but the impressions received through sensation, or the ideas left by them, he considered that a word which had not a meaning to be found in either of these things, had no meaning at all. He looked upon ideas as the goods with which the mind was stored; and on these stores, as being of the character of impressions, while they were in the state of coming into the mind. When any one, then, in reasoning, or any other kind of literature, spoke of any thing as existing, the principle of his theory was, that this storehouse of idealized impressions should be searched for one corresponding to the term made use [72]of. If such an impression were not found, the word was, so far as our human faculties were concerned, an unmeaning one. Whether there was any existence corresponding to its meaning, no one could say: all that the sceptical philosopher could decide was, that, so far as human intellect was put in possession of materials for thought, it had nothing to warrant it in saying, that this word represented any thing of which that intellect had cognizance.

This limitation of the material put at the disposal of the mind, was largely illustrated in the course of the work; and the illustrations assumed some such character as this:—Imaginative writers present us with descriptions of things which never, within our own experience, have existed,—of things which, we believe, never have had existence. Yet, however fantastic and heterogeneous may be the representations thus presented to our notice, there is no one part, of which we form a conception, that is any thing more than a new arrangement of ideas that have been left in the mind by impressions deposited there by sensation. The most extravagant of eastern or classical fictions there find their elements. If it be a three-headed dog, a winged horse, a fiery dragon, or a golden palace, that is spoken of, the reader who forms a conception of the narrative puts it together with the ideas left in his mind by impressions conveyed through the external senses. If a spectre is said to be raised, it may be spoken of as not denser than the atmosphere, yet the attributes that bring a conception of it to the intellect are the form and proportions of a human being,—expression, action, and habiliments: all elements the ideas of which the mind has received through the impressions of the senses. If words were used in a book of fiction which did not admit of being thus realized by the [73]mind putting together a corresponding portion of the ideas stored up within it—supplying, as it were, the described costume from this wardrobe—then, according to Hume's philosophy, the word would be a sound without meaning. He maintained a like rule as to books of philosophy. If the authors used terms which were not thus represented in the storehouse of the matter of thought and language, they were not reasoning on what they knew; they were not using words as the signs of things signified, but printing unmeaning collections of letters, or uttering senseless sounds.

The system, if it were to be classed under the old metaphysical divisions, was one of nominalism. Such words as shape, colour, hardness, roughness, &c. the author of the Treatise could only admit to have a meaning in as far as they signified ideas in the mind; and these ideas could only be there as the relics of impressions derived through the senses. Thus, general terms, such as the categories of Aristotle, could have no existence except in so far as they represented and called up particulars. Of the abstract term colour, our notion is derived solely from the ideas left in the mind by the actual impressions made through the senses. Heat, cold, and largeness, so far as these words represent what is really in the mind, have no other foundation.

The application of this system to the mathematics, and to natural philosophy, was so startling as to afford to some readers almost a reductio ad absurdum. The infinite divisibility of matter was arraigned by Hume as so far from being a truth, that it was not even capable of being conceived by the mind, which had never yet received any impressions through the senses corresponding to the expression. Every man had seen matter divided—some into smaller fragments [74]than others; but where our ideas, derived from actual experiment, stopped in minuteness of division, the conception of divisibility stopped also. The truth of geometrical demonstration, as applicable to practice, he did not deny; but he maintained, or rather seemed to maintain, for his reasoning here is of a highly subtle order, that we have a conception of these operations only in as far as they concur with really existing things, or, more properly speaking, with the ideas in the mind conveyed thither by the senses. Of the point, which has no breadth, depth, or length; of the straight line, which is deficient in the first and second, and not in the last of these qualities, he denied that we could have an idea, unless that idea were just as much the representative of an actual existence as any other idea is.

Infinity of space was an expression to which he had an objection on similar grounds; it had no idea corresponding to it lodged in the mind. Of space finite in various quantities, the mind possessed ideas stored up from repeated impressions, and by adding these ideas together, more or less vastness in the conception of finite space was afforded. But any thing beyond this definitive increase, attested as it was by the senses, the mind had no means of conceiving. Whatever might be in another intellectual world, there was no idea corresponding to infinity of space in the mind of man. It thence followed, that space unoccupied was a conception of which the mind was incapable, because the impressions originally conveyed to the mind were the medium through which the conception of space existed, and where there were no ideas of such impressions, an aggregate idea of space was wanting. In the same manner it was held, that it was in a succession of impressions, with ideas corresponding, that [75]the conception of time consisted, and that without such a succession, time would be a thing unknown and unconceived. Our ideas of numbers he found to be but the collected ideas of the impressions of the units of which the senses have received distinct impressions; and in confirmation of this he appealed to the distinctness of our notion of small numbers, which our mind has been accustomed to find represented by units, and our imperfect conception of those large numbers, which we have never had presented to us in detail. How readily we have a notion of six, but how imperfectly the mind receives the conception of six millions; how clearly we perceive, in units, the difference between six and twelve, but how imperfect is our notion of the difference between six millions and twelve millions.[75:1]

[76]All human consciousness being of these two materials, impressions and ideas, the answer to the question, What knowledge have we of an external world, resolved itself into this, that there were certain impressions and ideas which we supposed to relate to it—further we knew not. When we turn, according to this theory, from the external world, and, looking into ourselves, ask what certainty we have of separate self-existence, we find but a string of impressions and ideas, and we have no means of linking these together into any notion of a continuous existence. Such is that boasted thing the human intellect, when its elements are searched out by a rigid application of the sceptical philosophy of Hume. Not a thing separate and self-existent, which was, and is, and shall continue; but a succession of mere separate entities, called in one view impressions, in another ideas.[76:1]