HKB
is sometimes doubt whether that which is supposed to be an impression may not be a vivid idea, and that which is supposed to be an idea a faint impression.
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 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 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.