But I think it is quite plain that he does not mean _merely_ this, when he says that truth is mutable. No one would choose this way of expressing himself if he merely meant to say that _some_ things are mutable. Truth, Professor James has told us, is a property of certain of our ideas. And those of our ideas, which are true or false, are certainly only a part of the Universe. Other things in the Universe might, therefore, change, even if our ideas never changed in respect of this property. And our ideas themselves do undoubtedly change in some respects. A given idea exists in my mind at one moment and does not exist in it at another. At one moment it is in my mind and not in somebody else's, and at another in somebody else's and not in mine. I sometimes think of the truth that twice two are four when I am in one mood, and sometimes when I am in another. I sometimes think of it in connection with one set of ideas and sometimes in connection with another set. Ideas, then, are constantly changing in some respects. They come and go; and at one time they stand in a given relation to other things or ideas, to which at another time they do not stand in that relation. In this sense, any given idea may certainly have a property at one time which it has not got at another time. All this seems obvious; and all this cannot be admitted, without admitting that reality is mutable--that _some_ things change. But obviously it does not seem to follow from this that there is _no_ respect in which ideas are immutable. It does not seem to follow that because ideas, and other things, change some of their properties, they necessarily change that one which we are considering--namely, "truth." It does not follow that a given idea, which has the property of truth at one time, ever exists at any other time without having that property. And yet that this _does_ happen seems to be part of what is meant by saying that truth is mutable. Plainly, therefore, to say this is to say something quite different from saying that _some_ things are mutable. Even, therefore, if we admit that _some_ things are mutable, it is still open to consider whether truth is so. And this is what I want now to consider. Is it the case that an idea which exists at one time, and is true then, ever exists at any other time, without being true? Is it the case that any idea ever changes from true to false? That it has the property of being true on one of the occasions when it exists, and that it has _not_ this property, but that of being false instead, on some other occasion when it exists?