On this gradualness in the changes of our mental content the principles of nerve-action can throw some more light. When studying, in Chapter III, the summation of nervous activities, we saw that no state of the brain can be supposed instantly to die away. If a new state comes, the inertia of the old state will still be there and modify the result accordingly. Of course we cannot tell, in our ignorance, what in each instance the modifications ought to be. The commonest modifications in sense-perception are known as the phenomena of contrast. In æsthetics they are the feelings of delight or displeasure which certain particular orders in a series of impressions give. In thought, strictly and narrowly so called, they are unquestionably that consciousness of the _whence_ and the _whither_ that always accompanies its flows. If recently the brain-tract _a_ was vividly excited, and then _b_, and now vividly _c_, the total present consciousness is not produced simply by _c_'s excitement, but also by the dying vibrations of _a_ and _b_ as well. If we want to represent the brain-process we must write it thus: __{a}b^{c}_--three different processes coexisting, and correlated with them a thought which is no one of the three thoughts which they would have produced had each of them occurred alone. But whatever this fourth thought may exactly be, it seems impossible that it should not be something _like_ each of the three other thoughts whose tracts are concerned in its production, though in a fast-waning phase. It all goes back to what we said in another connection only a few pages ago (p. 233). As the total neurosis changes, so does the total psychosis change. But as the changes of neurosis are never absolutely discontinuous, so must the successive psychoses shade gradually into each other, although their _rate_ of change may be much faster at one moment than at the next. * * * * *