Definition is either of the subject-kind or of the property that is grounded in it. Of the self-subsistent definition is [Greek: ousias tis gnôrismos][54] by exposition of genus and differentia.[55] It is indemonstrable. It presumes the reality of its subject in a postulate of existence. It belongs to the principles of demonstration. _Summa genera_ and groups below _infimae species_ are indefinable. The former are susceptible of elucidation by indication of what falls under them. The latter are only describable by their accidents. There can here be no true differentia. The artificiality of the limit to the articulation of species was one of the points to which the downfall of Aristotle's influence was largely due. Of a non-self-subsistent or attributive conception definition in its highest attainable form is a recasting of the syllogism, in which it was shown that the attribute was grounded in the substance or self-subsistent subject of which it is. Eclipse of the moon, e.g. is privation of light from the moon by the interposition of the earth between it and the sun. In the scientific syllogism the interposition of the earth is the middle term, the cause or "because" ([Greek: dioti]), the residue of the definition is conclusion. The difference then is in verbal expression, way of putting, inflexion.[56] If we pluck the fruit of the conclusion, severing its nexus with the stock from which it springs, we have an imperfect form of definition, while, if further we abandon all idea of making it adequate by exhibition of its ground, we have, with still the same form of words, a definition merely nominal or lexicographical. In the aporematic treatment of the relation of definition and syllogism identical as to one form and in one view, distinct as to another form and in another view, much of Aristotle's discussion consists. The rest is a consideration of scientific inquiry as converging in [Greek: mesou zêtêsis], the investigation of the link or "because" as ground in the nature of things. [Greek: To men gar aition to meson][57] real ground and thought link fall together. The advance from syllogism as formal implication is a notable one. It is not enough to have for middle term a _causa cognoscendi_ merely. We must have a _causa essendi_. The planets are near, and we know it by their not twinkling,[58] but science must conceive their nearness as the cause of their not twinkling and make the _prius_ in the real order the middle term of its syllogism. In this irreversible catena proceeding from ground to consequent, we have left far behind such things as the formal parity of genus and differentia considered as falling under the same predicable,[59] and hence justified in part Porphyry's divergence from the scheme of predicables. We need devices, indeed, to determine priority or superior claim to be "better known absolutely or in the order of nature," but on the whole the problem is fairly faced.[60]