It is the special aim of Philosophy to discover and systematise the abstract relations of things; and for this purpose it is forced to allow the things themselves to drop out of sight, fixing attention solely on the quality immediately investigated, to the neglect of all other qualities.  Thus the philosopher, having to appreciate the mass, density, refracting power, or chemical constitution of some object, finds he can best appreciate this by isolating it from every other detail.  He abstracts this one quality from the complex bundle of qualities which constitute the object, and he makes this one stand for the whole.  This is a necessary simplification.  If all the qualities were equally present to his mind, his vision would be perplexed by their multiple suggestions.  He may follow out the relations of each in turn, but he cannot follow them out together.

The aim of the poet is very different.  He wishes to kindle the emotions by the suggestion of objects themselves; and for this purpose he must present images of the objects rather than of any single quality.  It is true that he also must exercise a power of abstraction and selection, tie cannot without confusion present all the details.  And it is here that the fine selective instinct of the true artist shows itself, in knowing what details to present and what to omit.  Observe this:  the abstraction of the philosopher is meant to keep the object itself, with its perturbing suggestions, out of sight, allowing only one quality to fill the field of vision; whereas the abstraction of the poet is meant to bring the object itself into more vivid relief, to make it visible by means of the selected qualities.  In other words, the one aims at abstract symbols, the other at picturesque effects.  The one can carry on his deductions by the aid of colourless signs, X or Y. The other appeals to the emotions through the symbols which will most vividly express the real objects in their relations to our sensibilities.

Imagination is obviously active in both.  From known facts the philosopher infers the facts that are unapparent.  He does so by an effort of imagination (hypothesis) which has to be subjected to verification:  he makes a mental picture of the unapparent fact, and then sets about to prove that his picture does in some way correspond with the reality.  The correctness of his hypothesis and verification must depend on the clearness of his vision.  Were all the qualities of things apparent to Sense, there would be no longer any mystery.  A glance would be Science.  But only some of the facts are visible; and it is because we see little, that we have to imagine much.  We see a feather rising in the air, and a quill, from the same bird, sinking to the ground:  these contradictory reports of sense lead the mind astray; or perhaps excite a desire to know the reason.  We cannot see,—­we must imagine,—­the unapparent facts.  Many mental pictures may be formed, but to form the one which corresponds with the reality requires great sagacity and a very clear vision of known facts.  In trying to form this mental picture we remember that when the air is removed the feather fails as rapidly as the quill, and thus we see that the air is the cause of the feather’s rising; we mentally see the air pushing under the feather, and see it almost as plainly as if the air were a visible mass thrusting the feather upwards.