Under these circumstances, again, it would seem that the only impressions of relation which could arise would be those of co-existence and of similarity.

For succession implies memory of an antecedent state.[24] But the special peculiarity of the cerebral apparatus is, that any given function which has once been performed is very easily set a-going again, by causes more or less different from those to which it owed its origin.
Of the mechanism of this generation of images of impressions or ideas (in Hume's sense), which may be termed _Ideation_, we know nothing at present, though the fact and its results are familiar enough.
During our waking, and many of our sleeping, hours, in fact, the function of ideation is in continual, if not continuous, activity.
Trains of thought, as we call them, succeed one another without intermission, even when the starting of new trains by fresh sense-impressions is as far as possible prevented.

The rapidity and the intensity of this ideational process are obviously dependent upon physiological conditions.

The widest differences in these respects are constitutional in men of different temperaments; and are observable in oneself, under varying conditions of hunger and repletion, fatigue and freshness, calmness and emotional excitement.

The influence of diet on dreams; of stimulants upon the fulness and the velocity of the stream of thought; the delirious phantasms generated by disease, by hashish, or by alcohol; will occur to every one as examples of the marvellous sensitiveness of the apparatus of ideation to purely physical influences.
The succession of mental states in ideation is not fortuitous, but follows the law of association, which may be stated thus: that every idea tends to be followed by some other idea which is associated with the first, or its impression, by a relation of succession, of contiguity, or of likeness.
Thus the idea of the word horse just now presented itself to my mind, and was followed in quick succession by the ideas of four legs, hoofs, teeth, rider, saddle, racing, cheating; all of which ideas are connected in my experience with the impression, or the idea, of a horse and with one another, by the relations of contiguity and succession.