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. No great
attention to what passes in the mind is needful to prove that our trains
of thought are neither to be arrested, nor even permanently controlled,
by our desires or emotions. Nevertheless they are largely influenced by
them. In the presence of a strong desire, or emotion, the stream of
thought no longer flows on in a straight course, but seems, as it were,
to eddy round the idea of that which is the object of the emotion. Every
one who has "eaten his bread in sorrow" knows how strangely the current
of ideas w