to mere states of your own private mind. You are asking, as Kant expresses it, just this, viz., whether, “In the progress of possible experience, you would come to perceive the presence of such inhabitants?” An answer to that question is even now true or false. And the objects of the one boundless realm of possible experience, — a realm which the sciences of nature study, are real, precisely in so far as all such propositions, quite apart from your present empirical observation or mine, but not independently of the predetermined constitution of all experience, are even now true or false.
A quotation from Kant’s discussion of the second of his so-called Postulates of Empirical Thought (Kr. d. r. V. 2d edit., p. 273) will help to bring his thought before you in his own way. “Perception,” says Kant, “which gives to a concept its material embodiment, is the only test of actuality. But one can, nevertheless, in advance of the perception of an object, and consequently in a relatively a priori fashion, know the existence of this object, in case the thing in question is connected with any of our perceptions according to the principles of the empirical synthesis of phenomena (i.e. according to the law of Causality, one of the other fundamental principles). For then the existence of the things is linked with our percepts in a possible experience, and by virtue of our general principles we can pass from our actual perception to the thing in question by a series of possible experiences. Thus we may recognize the existence of a magnetic substance pervading all matter, by virtue of our perception of the magnetic attraction of iron, although an immediate perception of the magnetic matter is impossible
