Art can operate only upon the surface of things formed by nature, as wood, iron, stone, wool, and similar things; but nature operates from the centre so to speak, of its subject, or matter, which in itself is wholly devoid of form. The subjects of the arts are many--of nature one; for those being diversely formed by nature, are different and various, while the latter, not being formed at all, is entirely indifferent,--every difference and variety being due to the form.[269] As it is absolutely formless, this matter cannot be perceived by the senses, which are the media of natural forms, but only by the eye of reason. As visible matter, that of art, remains the same under countless variations of form,--the form of a tree becoming that of a trunk, of a beam, of a table, a chair, a stool, a comb, its nature as wood continuing throughout; so in _nature_ that which was seed becomes herb; the herb, corn in the ear; the corn, bread; the bread, bile; bile, blood; blood again seed, an embryo, a man, a corpse, earth, stone, or other things, and so through all natural forms. There must then be one and the same thing which in itself is not stone nor earth, nor corpse, nor man, nor embryo, nor blood, nor anything else.[270] So the Pythagorean Timaeus[271] inferred, from the transmutations of the elements one into another,--earth into water, the dry into the moist,--a _tertium quid_, which was neither moist nor dry, but became subject now of the one, now of the other _nature_. Otherwise the earth would have gone to nothing and the water come from nothing, which is impossible. Thus nothing is ever annihilated but the accidental, the exterior, material form, both matter and the substantial form, _i.e._ spirit, being eternal. [Sidenote: Natural forms.] The argument has proved that there is a something, the "I know not what" of Locke, which is the substance of all natural things, "natural forms." We have now to see in what relation this substance stands to the forms, the differences, which are on its surface. All natural forms dissolve in matter, and come again in matter, so that nothing is really "constant, firm, eternal, or deserving of the name of a principle, but matter: besides that the forms have no existence without matter, in it they are generated and decay, from it they issue, into it are received again; therefore matter, which remains always the same and always fruitful, must be regarded as the only substantial principle, as that which always is and always abides; and the forms but as varying dispositions of matter, which come and go, cease and are renewed; therefore they have no claim to be principles."[272]