Besides uniformities of succession, which always depend on causation, there are uniformities of coexistence. These also, whenever the coexisting phenomena are effects of causes, whether of one common cause or of several different causes, depend on the laws of their cause or causes; and, till resolved into these laws, are mere empirical laws. But there are some uniformities of coexistence, viz. those between the ultimate properties of _kinds_, which do not depend on causation, and therefore seem entitled to be classed as a peculiar sort of laws of nature. As, however, the presumption always is (except in the case of those _kinds_ which are called _simple substances_ or elementary natural agents), that a thing's properties really depend on causes though not traced, and we _never_ can be certain that they do not; we cannot safely claim (though it _may_ be an ultimate truth) higher certainty than that of an empirical law for any generalisation about coexistence, that is to say (since _kinds_ are known to us only by their properties, and, consequently, all assertions about them are assertions about the coexistence of something with those properties), about the properties of _kinds_. Besides, no rigorous inductive system can be applied to the uniformities of coexistence, since there is no general axiom related to them, as is the law of causation to those of succession, to serve as a basis for such a system. Thus, Bacon's practical applications of his method failed, from his supposing that we can have previous certainty that a property must have an invariable coexistent (as it must have an invariable antecedent), which he called its form. He ought to have seen that his great logical instrument, elimination, is inapplicable to coexistences, since things, which agree in having certain apparently ultimate properties, often agree in nothing else; even the properties which (e.g. Hotness) are effects of causes, generally being not connected with the ultimate resemblances or diversities in the objects, but depending on some outward circumstance. Our only substitute for an universal law of coexistence is the ancients' induction _per enumerationem simplicem ubi non reperitur instantia contradictoria_, that is, the improbability that an exception, if any existed, could have hitherto remained unobserved. But the certainty thus arrived at can be only that of an empirical law, true within the limits of the observations. For the coexistent property must be either a property of the _kind_, or an accident, that is, something due to an extrinsic cause, and not to the _kind_ (whose own indigenous properties are always the same). And the ancients' class of induction can only prove that _within given limits_, either (in the latter case) one common, though unknown, cause has always been operating, or (in the former case) that no new _kind_ of the object has _as yet_ or _by us_ been discovered.