[34] The metaphysical realist is faced by a further difficulty when he seeks to explain the similarity between the world pictures of different human individuals. He has to ask himself: How is it that the picture of the world which I build up out of my subjectively determined percepts and my concepts turns out to be the same as the one which another individual is also building up out of the same two subjective factors? How can I, in any case, draw conclusions from my own subjective picture of the world about that of another human being? The fact that people can understand and get on with one another in practical life leads the metaphysical realist to conclude that their subjective world pictures must be similar. From the similarity of these world pictures he then further concludes that the "individual spirits" behind the single human subjects as percepts, or the "I-in-itself" behind the subjects, must also be like one another.

[35] This is an inference from a sum of effects to the character of the underlying causes. We believe that we can understand the situation well enough from a sufficiently large number of instances to KNOW how the inferred causes will behave in other instances. Such an inference is called an inductive inference. We shall be obliged to modify its results if further observation yields some unexpected element, because the character of our conclusion is, after all, determined only by the particular form of our actual observations. The metaphysical realist asserts that this knowledge of causes, though conditional, is nevertheless quite sufficient for practical life.

[36] Inductive inference is the method underlying modern metaphysical realism. At one time it was thought that we could evolve something out of concepts that is no longer a concept. It was thought that the metaphysical realities, which metaphysical realism after all requires, could be known by means of concepts. This kind of philosophizing is now out of date. Instead it is thought that one can infer from a sufficiently large number of perceptual facts the character of the thing-in-itself which underlies these facts. Whereas formerly it was from concepts, now it is from percepts that people seek to evolve the metaphysical. Since one has concepts before oneself in transparent clearness, it was thought that one might be able to deduce the metaphysical from them with absolute certainty. Percepts are not given with the same transparent clearness. Each subsequent one is a little different from others of the same kind which preceded it. Basically, therefore, anything inferred from past percepts will be somewhat modified by each subsequent percept. The character of the metaphysical thus obtained can, therefore, be only relatively true, since it is subject to correction by further instances. Eduard von Hartmann's metaphysics has a character determined by this basic method, as expressed in the motto on the title page of his first important book: "Speculative results following the inductive method of Natural Science."