"The view of physical objects which I now adhere to is, I think, best explained in Human Knowledge. I think that what occurs in any small region of physical space is a complex of spatially overlapping occurrences. In physics this complex can be treated as a single 'thing'—namely a piece of matter—but in dealing with perception a different way of dealing with the complex is necessary. From any given region of physical space causal lines radiate having a certain degree of structural similarity with some constituent of the complex occurring in that place. Many such causal lines meet at any region in physical space. If such a region is a brain, they constitute the complex of what that brain has 'perceived'. This view differs in some respects from that put forward in Our Knowledge of the External World."