In order to answer this question clearly, it is, I think, necessary to make still another distinction. It does certainly seem to be true, _in a sense_, that a given idea may be true on one occasion and false on another. We constantly speak as if there were cases in which a given thing was true on one occasion and false on another; and I think it cannot be denied that, when we so speak, we are often expressing in a perfectly proper and legitimate manner something which is undeniably true. It is true now, I might say, that I am in this room; but to-morrow this will not be true. It is true now that men are often very miserable; but perhaps in some future state of society this will not be true. These are perfectly natural forms of expression, and what they express is something which certainly may be true. And yet what they do apparently assert is that something or other, which is true at one time, will not, or _perhaps_ will not, be true at another. We constantly use such expressions, which imply that what is true at one time is not true at another; and it is certainly legitimate to use them. And hence, I think, we must admit that, _in a sense_, it is true that a thing may be true at one time which is not true at another; in that sense, namely, in which we use these expressions. And it is, I think, also plain that these things, which may be true at one time and false at another, may, _in a sense,_ be ideas. We might even say: The idea that I am in this room, is true now; but to-morrow it will not be true. We might say this without any strain on language. In any ordinary book--indeed, in any philosophical book, where the subject we are at present discussing was not being expressly discussed--such expressions do, I think, constantly occur. And we should pass them, without any objection. We should at once understand what they meant, and treat them as perfectly natural expressions of things undeniably true. We must, then, I think, admit that, _in a sense_, an idea may be true at one time, and false at another. The question is: In what sense? What is the truth for which these perfectly legitimate expressions stand?