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Memory-Train.

Now, when you are trying to recall a succession of observed impressions, the images pertaining to these are observed as if they were actually arranged in an order corresponding to the order in which the original impressions were received. This supposed arrangement is called, as everybody knows, the 'Memory-Train', and it is noticeable that the process of remembering events in the order in which they occurred is one which involves sometimes a very considerable mental effort. But if you are merely allowing your mind to wander—as in a daydream—without knowingly aiming at any definite goal, the set of images which is then observed appears to be arranged in a sequence which has little correspondence with any previous observed succession of events.

Train of Ideas.

This curious succession of images is called the 'Train of Ideas', and it is possibly a very significant fact that the simple, undirected following of a train of ideas appears to entail no mental effort or fatigue whatsoever.

Almost everybody has, at one time or another, amused himself by retracing the train of ideas which has led him, without any conscious aim on his part, to think of, or remember, a certain thing. 'I saw this', he will say, 'and that made me remember So-and-so; and that made me think of such-and-such.' And so on. Here, however, is a specific example.

It is now evening, and in front of me stands a teacup with a chequered black and white bordering. The sight of this (an impression) 'brings up' a memory image of the chequered oilcloth floor-covering which, this morning, I was using as material for an experiment in obtaining after-impressions. Now, at the time of making that experiment I was thinking of Ward's description of these phenomena in the Encyclopædia Britannica; and the next image to appear before me is an image of the red volume in question (mine is the small-print edition). Following that, there appears an image of an open page in the volume, and a very vivid image of the sensation of eye-strain involved in its reading. That 'brings up' an image of the reading-glass I sometimes use. That 'brings up' the image of the lens I borrowed in a fishing-tackle shop yesterday morning in order to examine some trout flies I was buying. That 'brings up' the image of the friend for whom I had purchased those flies, as he stood when asking me to do so. And that 'brings up' the pleasing image of the two-and-a-half pound trout I annexed from that friend's water two days ago. Thus, starting with a teacup, I arrive at a trout.

Now, examination of the nature of a train of ideas brings to light the following facts.

Generic Images.

When a number of partly similar impressions have been attended to at different times, there is observable, besides the several memory images pertaining to those several impressions, a vague, general image comprising nothing beyond the key elements which are common to all those separate images. For example, the images of the hundreds of tobacco pipes which I have seen, smoked, and handled, all contain a common element which is now apparent to me as an ill-defined image of 'pipe' in general. It presents all the essential characteristics which serve to distinguish a pipe from any other article such as, say, an umbrella. Such characteristics are: hollow bowl, tubular stem—in short, an appearance of utility for the purpose of smoking. But this indefinite image does not exhibit any indication of specific colour or precise dimensions. It seems, however, to be the nucleus of all the definite images of particular pipes to be found in my mental equipment; for, if attention be directed to it, there will quickly become observable the image of sometimes one and sometimes another of such particular pipes.

These vague, almost formless general images are called 'Generic Images', and they appear to be analogous to a central knot to which the specific, definite images are in the relation of radiating threads.

Associational Network.

It is obvious that many of these threads—these definite images—may be radiating also from another generic image. A definite image of a particular wooden pipe-bowl may pertain, on one side, to the generic image 'pipe', and, on another, to the generic image I call 'grained wood'. That generic image may have, as another of its components, a definite image of a polished walnut table, which image, again, may also be a radiating thread pertaining to the generic image 'furniture'. A thread from 'furniture'—say, the image of a particular suite seen in a shop window—may be the link with the generic image 'antiquities'. So far, then, we are confronted with something analogous to a network of knots (generic images) and radiating threads (definite images) along the meshes of which attention may be led without conscious effort on the part of the observer. Ideas linked together in this manner graphically analogous to a network of knots and threads are said to be 'associated'. Hence we may refer to the structure in question as the 'Associational Network'.

It is commonly assumed that association is of two kinds: association by similarity, as when one event recalls a similar event which may have happened long ago; and association by contiguity, which means that, when two events have occurred in close succession, the recalling of one leads to the recollection of the other.

To the physiologist the associational network is simply the network of brain-paths, the 'knots' being regions—or patterns—therein, and the 'connecting threads' being paths which pertain to more than one such region—or pattern. All the phenomena of association seem to be adequately accounted for on that supposition; and on no other theory, so far as I can see, is it possible to account for association by 'similarity' at all.

In the absence of any other guidance, the path taken by the train of ideas seems to be conditioned very largely by the factor of freshness in the images. Other things being equal, an image which has been recently established makes a stronger bid for the wandering attention than does one which has long been neglected. The reader will notice that, in the example of a train of ideas given a little way back (the one which began with a teacup), all the images related to experiences which had recently occurred. For example, the black and white chequering of the teacup led me, not to chess, which is a very obtrusive generic image of mine, but to the piece of linoleum I had seen that morning. Physiologically, this would mean that brain-paths which have been recently traversed offer a better passage to the currents of nervous energy than do those which have been allowed to fall into disuse.

The supposed 'memory-train' does not appear to be anything more than a particular pathway through the associational network, the pathway which happens to have been thus recently traversed. If you try to trace a 'memory-train' back for more than a little way, you find that the path has ceased to be clearly marked out: the images do not come up in a steadily correct sequence of, so to say, their own accord. You have to help the memory out by reasoning as to which event must have happened next—and sometimes you reason wrongly.

Dreams.

Dreams, like many other mental phenomena, are composed largely of images supplied by an associational network. But they differ from mind-wandering in several important respects. In the latter form of activity reason is nearly always partially at work to determine the course to be followed along the network. But in dreams this guidance seems to be largely lacking, and the dream images present themselves as real—though curiously unstable—episodes in a personal adventure story of an only partially reasonable character.

Integration.

Association between the dream-images is sometimes clear enough; but, as a general rule, such association takes the curious form known as an 'Integration'. By this word we shall mean: 'A combination of associated images in which the composing elements are qualitatively distinguishable.' (This definition is from Baldwin's Dictionary of Philosophy and Psychology.) For example, the image of a pink dress seen in a shop window on Monday, and that of a shop girl seen when the same place is re-visited on Tuesday, may combine, in Tuesday night's dream, into a single image of the shop girl wearing the pink dress. But on waking and recalling the dream, the two components of the dream-image, dress and girl, are clearly distinguishable as images of originally separate impressions.

Concepts.

It will be noticed that, in the foregoing list of definitions, no attempt has been made to delve below that class of thought-process which is styled 'imagery'—a class in regard to which the psychoneural connection suggests itself very readily. Thought processes of a higher order are not yet properly—or even, perhaps, improperly—understood. Our knowledge of these is of the very vaguest description. There appear to be certain generalized ideas called 'Concepts', such as, for example, those we employ when we think of 'eating', 'playing', 'imagining', or of 'difficulty', 'truth', 'deception', 'difference'; but it is even doubtful whether these may legitimately be herded together under any such single class-name. Compare, for instance, 'eating' with 'difference'. The former idea may be no more than the stimulation of the more broadly determinative lines of some extensive pattern in the plexus of brain-paths; but the latter may claim a connection with, or share in, every single idea we can formulate.

It is here that the animist is enabled to put up his best fight in defence of the observer's alleged power of intervention. But even here the materialist may claim to have overrun a considerable part of the disputed territory. For the man whose brain has been injured by disease may, apparently, forget what 'eating' is; or may be more than a little hazy regarding the existence of a 'difference' betwixt himself and a grasshopper.

Our present pathway does not take us across this particular battlefield; though we pass within hailing distance of the combatants. From them, however, we may accept the information that concepts are often determinants of the route that attention follows through the associational net. It is hardly possible for the unguided attention to dwell upon any concept without finding itself, a moment later, confronted by a generic or even specific, image clearly related to that main idea.