The assumptions which determine Avenarius’ central problem may, if somewhat freely stated, be expressed in the following manner. Nothing exists save experience; and the fundamental characteristic of the content of experience is space. The self apprehends itself as an embodied existence, and so as spatially related to the objects around it. All its perceptions, thoughts and feelings, have reference direct or indirect either to the body or to its environment. Now the spatial world thus experienced varies together with one particular part of itself, namely, with the brain. And this relation is mutual; change in either involves change in both; they stand in functional relation, varying simultaneously with one another. Since nothing exists save as experienced, and since as experienced it involves change in the brain, the relation must be of this nature. On the other hand, however, objects are causally related to the brain, and by their changes produce changes in it. This causal relation as involving sequence and implying independent self-centred existence holds only in the forward order, and therefore excludes the possibility of simultaneous variation. The fundamental problem of metaphysics is to reconcile these two standpoints, the attitude of pure experience with the standpoint adopted in physics and physiology. How can the whole vary simultaneously with a part of itself, and with a part which is causally dependent for its changes upon its relations to the rest of that whole? Avenarius will have nothing to do with the solution offered by subjective idealism—that our experience as purely subjective may vary simultaneously with those brain-states which real external objects have produced. That solution rests on a dualism which Avenarius denounces as ungrounded and absolutely false. Our experienced world is reality, and its functional relation to its own component, the brain, must therefore be reconcilable in some other
