exactly similar to a thing which does possess it, even though, in fact, there has not and won't be any such thing, that thing would have possessed or would possess the kind of value in question in exactly the same degree. It is essential to this meaning of "impossibility" that it should entitle us to assert what _would_ have been the case, under conditions which never have been and never will be realised; and it seems obvious that no mere empirical generalisation can entitle us to do this. But (2) to say that I am not using 'necessity' in this first sense, is by no means sufficient to explain what I do mean. For it certainly seems as if causal laws (though this is disputed) do entitle us to make assertions of the very kind that mere empirical generalisations do not entitle us to make. In virtue of a causal law we do seem to be entitled to assert such things as that, if a given thing had had a property or were to have a property F which it didn't have or won't have, it _would_ have had or _would_ have some other property G. And it might, therefore, be thought that the kind of 'necessity' and 'impossibility' I am talking of is this kind of causal 'necessity' and 'impossibility.' It is, therefore, important to insist that I do _not_ mean this kind either. If this were all I meant, it would again be by no means obvious, that purely subjective predicates might not satisfy our second condition. It may, for instance, for all I know, be true that there are causal laws which insure that in the case of everything that is 'beautiful,' anything exactly like any of these things would, in this Universe, excite a particular kind of feeling in everybody to whom it were presented in a particular way: and if that were so, we should have a subjective predicate which satisfied the condition that, when a given thing possesses that predicate, it is impossible (in the causal sense) that any exactly similar thing should not also possess it. The kind of necessity I am talking of is not, therefore, mere causal necessity either. When I say that if a given thing possesses a certain degree of intrinsic value, anything precisely similar to it _would_ necessarily _have_ possessed that value in exactly the same degree, I mean that it _would_ have done so, even if it had existed in a Universe in which the causal laws were quite different from what they are in this one. I mean, in short, that it is _impossible_ for any precisely similar thing to possess a different value, in precisely such a sense as that, in which it is, I think, generally admitted that it is _not_ impossible that causal laws should have been different from what they are--a sense of impossibility, therefore, which certainly does not depend merely on causal laws.