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. 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.