When agriculture, commerce, or manual crafts have given men some knowledge of nature, the world thus recognised and dominated is far from seeming ultimate. It is thought to lie between two others, both now often called mental, but in their original quality altogether disparate: the world of spiritual forces and that of sensuous appearance. The notions of permanence and independence by which material objects are conceived apply also, of course, to everything spiritual; and while the dominion exercised by spirits may be somewhat precarious, they are as remote as possible from immediacy and sensation. They come and go; they govern nature or, if they neglect to do so, it is from aversion or high indifference; they visit man with obsessions and diseases; they hasten to extricate him from difficulties; and they dwell in him, constituting his powers of conscience and invention. Sense, on the other hand, is a mere effect, either of body or spirit or of both in conjunction. It gives a vitiated personal view of these realities. Its pleasures are dangerous and unintelligent, and it perishes as it goes. [Sidenote: Spirit and sense defined by their relation to nature.] Such are, for primitive apperception, the three great realms of being: nature, sense, and spirit. Their frontiers, however, always remain uncertain. Sense, because it is insignificant when made an object, is long neglected by reflection. No attempt is made to describe its processes or ally them systematically to natural changes. Its illusions, when noticed, are regarded as scandals calculated to foster scepticism. The spiritual world is, on the other hand, a constant theme for poetry and speculation. In the absence of ideal science, it can be conceived only in myths, which are naturally as shifting and self-contradictory as they are persistent. They acquire no fixed character until, in dogmatic religion, they are defined with reference to natural events, foretold or reported. Nature is what first acquires a form and then imparts form to the other spheres. Sense admits definition and distribution only as an effect of nature and spirit only as its principle. [Sidenote: Vague notions of nature involve vague notions of spirit.]