We may pass from the case of the mental life of a fellow-man to the case of unperceived physical reality. A recent realist philosopher, Mr. L. T. Hobhouse, has brought forward as a clear instance of an independent physical reality, the case of a railway train just emerging from a tunnel. I do not perceive the train, he says, until it issues from the tunnel, but it was just as real while it was running through the tunnel. Its reality is therefore independent of the question whether it is perceived or not. But, in the first place, the argument requires that the train shall be empty; it must be a runaway train without driver, guard, or passengers, if the conditions presumed in the premisses are to be fulfilled. And, in the second place, we may retort that even an empty runaway train must have been despatched from somewhere by somebody. It must stand in some relation to the general scheme of purposes and interests expressed in our system of railway traffic, and it is precisely this connection, with a scheme of purposes and interests, which makes the runaway train a reality and not a mere fiction of an ingenious philosopher’s imagination. If Mr. Hobhouse’s argument proves the independent reality of the train in the tunnel, it ought equally to prove, and does equally prove, the independent existence of Selkirk’s fluctuations from hope to despair and back again on his isolated rock. And precisely because it proves both conclusions equally, the sort of independence it establishes cannot be independence of experience. Like all realist arguments, it turns on the identification of experience with the cognitive aspect of experience, an identification too often suggested by the language of the “idealists” themselves.[48] § 8. The persistent vitality of Realism is due to its protest against the fallacies of an opposing theory which has of late especially found favour with some distinguished students of natural science, and which we may conveniently call Subjectivism.[49] Realism, as we saw, started from the true premisses, that there are real facts of which my experience does not make me explicitly aware, and that my cognition even of my own experience is incomplete, and argued to the false conclusion that there are therefore realities independent of any experience. Subjectivism reasons in the opposite way. It asserts truly that there is no reality outside experience, and then falsely concludes that I can know of no reality except my own cognitive states. Its favourite formula are expressions such as, “We know only the modifications of our own consciousness,” “All we know is our own perceptions,” “Nothing exists but states of consciousness.” These formulæ are not all obviously identical in meaning, but the exponents of Subjectivism seem to use them without any conscious distinction, and we shall probably do the theory no injustice if we criticise it on the assumption that the expressions are meant to be of identical signification.