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I know that there is space? It is by comparison of mental experiences. All things were apparently spread before me as if on an immediately tangible plane surface or wall, until I began as an infant to test their relations. The simplest interpretation of space is the result of much mental experience, and it is impossible to dissociate space from the idea of it.

Yes, the materialist admits, but this is merely the training of the organism, and of course that is essential. Varied spatial experiences awaken varied ideas concerning the relationship of objects. Thus the ideas are produced by physical phenomena.
 
That is half of the truth, I reply, but do the varied experiences compare themselves? Do they fall into an adjustment such that I always know how to judge the connection of objects in space? What are the sensations by which I judge, and how are they known? What is the ego that feels, knows, and judges? Is that a mere automaton? The materialist is compelled to admit that the nature of the ego is unknown to him. He must admit that to touch different objects would not suffice to show that they are unequally distant from the tactual organism. This mental discovery is unlike anything the materialist can point to in the physical world. The existence of space is a discovery made by the mind. Likewise