TO WHAT CEREBRAL PROCESS IS THE SENSE OF TIME DUE?
Now, to what element in the brain-process may this sensibility be due? It cannot, as we have seen, be due to the mere duration itself of the process; it must be due to an element present at every moment of the process, and this element must bear the same inscrutable sort of relation to its correlative feeling which all other elements of neural activity bear to their psychic products, be the latter what they may. Several suggestions have been made as to what the element is in the case of time. Treating of them in a note,[557] I will try to express briefly the only conclusion which seems to emerge from a study of them and of the facts—unripe though that conclusion be.
The phenomena of 'summation of stimuli' in the nervous system prove that each stimulus leaves some latent activity behind it which only gradually passes away. (See above, [pp. 82-85].) Psychological proof of the same fact is afforded by those 'after-images' which we perceive when a sensorial stimulus is gone. We may read off peculiarities in an after-image, left by an object on the eye, which we failed to note in the original. We may 'hark back' and take in the meaning of a sound several seconds after it has ceased. Delay for a minute, however, and the echo itself of the clock or the question is mute; present sensations have banished it beyond recall. With the feeling of the present thing there must at all times mingle the fading echo of all those other things which the previous few seconds have supplied. Or, to state it in neural terms, there is at every moment a cumulation of brain-processes overlapping each other, of which the fainter ones are the dying phases of processes which but shortly previous were active in a maximal degree. The amount of the overlapping determines the feeling of the duration occupied. What events shall appear to occupy the duration depends on just what processes the overlapping processes are. We know so little of the intimate nature of the brain's activity that even where a sensation monotonously endures, we cannot say that the earlier moments of it do not leave fading processes behind which coexist with those of the present moment. Duration and events together form our intuition of the specious present with its content.[558] Why such an intuition should result from such a combination of brain-processes I do not pretend to say. All I aim at is to state the most elemental form of the psycho-physical conjunction.
I have assumed that the brain-processes are sensational ones. Processes of active attention (see Mr. Ward's account in [Footnote 556]) will leave similar fading brain-processes behind. If the mental processes are conceptual, a complication is introduced of which I will in a moment speak. Meanwhile, still speaking of sensational processes, a remark of Wundt's will throw additional light on the account I give. As is known, Wundt and others have proved that every act of perception of a sensorial stimulus takes an appreciable time. When two different stimuli—e.g. a sight and a sound—are given at once or nearly at once, we have difficulty in attending to both, and may wrongly judge their interval, or even invert their order. Now, as the result of his experiments on such stimuli. Wundt lays down this law:[559] that of the three possible determinations we may make of their order—
"namely, simultaneity, continuous transition, and discontinuous transition—only the first and last are realized, never the second. Invariably, when we fail to perceive the impressions as simultaneous, we notice a shorter or longer empty time between them, which seems to correspond to the sinking of one of the ideas and to the rise of the other.... For our attention may share itself equally between the two impressions, which will then compose one total percept [and be simultaneously felt]; or it may be so adapted to one event as to cause it to be perceived immediately, and then the second event can be perceived only after a certain time of latency, during which the attention reaches its effective maximum for it and diminishes for the first event. In this case the events are perceived as two, and in successive order—that is, as separated by a time-interval in which attention is not sufficiently accommodated to either to bring a distinct perception about.... While we are hurrying from one to the other, everything between them vanishes in the twilight of general consciousness."[560]
One might call this the law of discontinuous succession in time, of percepts to which we cannot easily attend at once. Each percept then requires a separate brain-process; and when one brain-process is at its maximum, the other would appear perforce to be in either a waning or a waxing phase. If our theory of the time-feeling be true, empty time must then subjectively appear to separate the two percepts, no matter how close together they may objectively be; for, according to that theory, the feeling of a time-duration is the immediate effect of such an overlapping of brain-processes of different phase—wherever and from whatever cause it may occur.
To pass, now, to conceptual processes: Suppose I think of the Creation, then of the Christian era, then of the battle of Waterloo, all within a few seconds. These matters have their dates far outside the specious present. The processes by which I think them, however, all overlap. What events, then, does the specious present seem to contain? Simply my successive acts of thinking these long-past things, not the long-past things themselves. As the instantly-present thought may be of a long-past thing, so the just-past thought may be of another long-past thing. When a long-past event is reproduced in memory and conceived with its date, the reproduction and conceiving traverse the specious present. The immediate content of the latter is thus all my direct experiences, whether subjective or objective. Some of these meanwhile may be representative of other experiences indefinitely remote.
The number of these direct experiences which the specious present and immediately-intuited past may embrace measures the extent of our 'primary,' as Exner calls it, or, as Richet calls it, of our 'elementary' memory.[561] The sensation resultant from the overlapping is that of the duration which the experiences seem to fill. As is the number of any larger set of events to that of these experiences, so we suppose is the length of that duration to this duration. But of the longer duration we have no direct 'realizing sense.' The variations in our appreciation of the same amount of real time may possibly be explained by alterations in the rate of fading in the images, producing changes in the complication of superposed processes, to which changes changed states of consciousness may correspond. But however long we may conceive a space of time to be, the objective amount of it which is directly perceived at any one moment by us can never exceed the scope of our 'primary memory' at the moment in question.[562]