Philosophical thought and knowledge, with that diversity of illustration and variety in method which follows from its universality, is, in this respect, somewhat in the same case with poetry. Of all the imitative arts poetry alone embraces, and by its nature is intended to embrace, the whole man. It is, therefore, free to borrow its similes or colors, and manifold figurative expressions, from every sphere of life and nature, and to take them now from this, now from that object, as on each occasion appears most striking and appropriate. Now, no one would think of prescribing unconditionally to poetry, and compelling her to take all her similes and figures either from flowers and plants, or from the animal world, or exclusively from any one of the several pursuits of man--from the sailor’s life, for instance, or the shepherd’s, or the huntsman’s--or from any of his handicrafts or mechanical arts. For although all such similes, and colors, and expressions, appropriately, introduced, are equally allowable in every poetical composition, and none of them need be rejected, still the exclusive use of any one class of them as a law would hamper the free poetic spirit and extinguish the living fancy. In the same way, philosophy is not confined to any one invariable and immutable form. At one time it may come forward in the guise of a moral, legislative, or a judicial discussion; at another, as a description of natural history. Or, perhaps, it may assume the method of an historical and genealogical development and derivation of ideas as best fitted to exhibit the thoughts which it aims at illustrating in their mutual coherence and connection. On other occasions, perhaps, it will take the shape of a scientific investigation of nature--of an experiment in a higher physiology--in order to test the existence of the invisible powers which it is its purpose to establish. Or again, by the employment of an algebraic equation, or of a mathematical form (which, however, it regards as nothing more than a symbol and visible hieroglyphic for a higher something that is invisible), it will, perhaps, most conveniently attain to its loftier aim. Every method and every scientific form is good; or, at least, when rightly employed, is good. But no one ought to be exclusive. No one must be carried out with painful uniformity, and with wearying monotony be invariably followed throughout.