Page:Philosophical Review Volume 1.djvu/168

152 mind. Our mental states are correlated immediately with brain states, it is true; but, more remotely, they are correlated with many other physical events, peripheral nerve currents for example, and the physical stimuli which occasion these. Of these latter correlations we have an extensive body of rather orderly knowledge. And, after all, may we not exaggerate the degree of our ignorance of brain states themselves? We don't know exactly what a nerve current is, it is true; but we know a good deal about it. We know that it follows a path, for instance, and consumes a fraction of a second of time in doing so. We know that, physically considered, our brain is only a mass of such paths, which incoming currents must somehow make their way through before they run out. We even know something about the consciousness with which particular paths are specially 'correlated,' those in the occipital lobes, e.g., being connected with the consciousness of visible things. Now the provisional value of such knowledge as this, however inexact it be, is still immense. It sketches an entire programme of investigation, and defines already one great kind of law which will be ascertained. The order in time of the nerve currents, namely, is what determines the order in time, the coexistences and successions of the states of mind to which they are related. Professor Ladd probably does not doubt the nerve-current theory of motor habits; he probably does not doubt that our ability to learn things 'by heart' is due to a capacity in the cerebral cortex for organizing definitely successive systems of paths of discharge. Does he then see any radical reason why the special time-order of the 'ideas' in any case whatever of 'association' may not be analogously explained? And if not, may he not go on to admit that the most characteristic features of our faculty of memory, of our perception of outer things, of our liability to illusion, etc., are most plausibly and naturally explained by acquired organic