What are the reasons for these differences? I reduce them to two principal ones, which I may briefly state thus—
The revivability of an impression is in direct ratio to its complexity, and consequently in inverse ratio to its simplicity.
The revivability of an impression (with certain exceptions to be mentioned afterwards) is in direct ratio to the motor elements included in it.
1. It is an incontestable fact that an isolated state of consciousness, with no relation to what precedes, accompanies, or follows it, has small chance of fixing itself in the memory. I hear a word of an unknown language, it immediately vanishes; but if I read and write it, if I associate it with some object or with various circumstances, it is fixed. It is easier to remember a group or a series than an isolated and unrelated term. Now, by their very nature, the visual images arrange themselves in complex aggregates, and the auditive in sequences (or even, in the case of harmony, in simultaneities), while the motor images are associated in series, every term of which awakens and brings with it the last. They accordingly fulfil the conditions of immediate and easy revivability. It is the same with pleasures, pains, emotions. Always connected with intellectual states (perceptions, representations, or ideas), they form part of an aggregate, and are involved in its resurgent movement.
The case is different with the images of our third group. They are not associated with one another; they have an isolated and individual character; they contract no relations, either of space or time, among themselves.
Let us take the case of odours. One excludes another; they are not associated in the imagination as visual images are in the recollection of a beautiful landscape. One of my correspondents is able to recall at will the scent of pinks; she tried to do so while walking in a wood full of decaying leaves and their smell, but without success; one odour excluded the other. Neither can they be arranged in sequences. I am aware that an English chemist, Piesse, has claimed the ability to class scents in a continuous series, like notes, patchouli corresponding to lower c in the key of F, and civet to the upper f in the key of G—the whole in tones and semitones; but no one, so far as I know, has taken this fancy seriously.
In the same way, tastes may be associated with other images, as of hunger (I have collected several cases of this), which makes their revival more difficult. Among themselves they do not form associations, but combinations. If associations ever occur they are extremely rare and limited in character.
Hunger and thirst are special, indecomposable states. Disgust and fatigue are revived easily enough, and, as we have seen, by almost every one; but it must be remembered that these states are composed of somewhat heterogeneous—sensory and motor—elements, and that they approach the character of aggregates.
This antithesis between the first two groups and that of odours, tastes, and internal sensations, depends, no doubt, on certain physiological conditions. As we can do nothing but hazard conjectures on this point, it is better to abstain.
2. The second theory stated above—viz., that the revivability is in direct ratio to the motor elements included in the image—is more open to question. I only give it as a partial, secondary, subsidiary explanation, applicable to many cases but not to all, and allowing of numerous exceptions. Since we have to do with an empirical law—a pure generalisation from experience—we must test it by facts, in order to fix its bearing and value. This rapid examination will justify my restrictions and reservations.