Let us add some illustrations from philosophy. Some philosophers hold, in common with general opinion, that sense experience is caused by physical objects; others hold that there are no physical objects, but that consciousness is the one and only reality; and there are others who think that the reality that gives rise to our sense experience is neither physical in the sense of a material thing, nor mental in the sense of consciousness or thought, but is movement or change--change that requires no support and is absolute. All these are opinions, and may be false, and our belief that any one of them is true does not depend on immediate experience, but on reasons. The best that can be said in favour of any belief is that there is no reason for supposing it false, and the worst that can be said against any belief is that there is no reason for supposing it true. Our problem, then, is to know what constitutes the nature of error in any one of these examples if it is, as each one may be, false? The instances we have given are all of them propositions or judgments, or else conceptions formed out of propositions or judgments, the purpose of which is to interpret experience. The actual experience itself, in so far as it consists of the actual presence of the object to the mind aware of it, is, as we have seen, neither truth nor error; it simply is what it is. It is the conceptions by which we interpret this experience that are {79} true or false. And our problem is that the meaning or content of a conception, that which is present to the mind when we make a judgment, is precisely the same whether the conception is true or false, there is no distinctive mark or feature by which we can know that in the one case the object of thought is a real or actual fact, in the other an opinion to which no reality corresponds. And, further, it seems exceedingly difficult to understand in what way a non-reality can be present to the mind at all.