nferential association. Many of our inferences, like the
quasi-instinctive impulses which they accompany and modify, take place
when we are making no conscious effort at all. In such a purely
instinctive action as leaping backwards from a falling stone, the
impulse to leap and the inference that there is danger, are simply two
names for a single automatic and unconscious process. We can speak of
instinctive inference as well as of instinctive impulse; we draw, for
instance, by an instinctive mental process, inferences as to the
distance and solidity of objects from the movements of our eye-muscles
in focussing, and from the difference between the images on our two
retinas. We are unaware of the method by which we arrive at these
inferences, and even when we know that the double photograph in the
stereoscope is flat, or that the conjurer has placed two converging
sheets of looking-glass beneath his table, we can only say that the
photograph 'looks' solid, or that we 'seem' to see right under the
table.

The whole process of inference, rational or non-rational, is indeed
built up from the primary fact that one mental state may call up
another, either because the two have been associated together in the
history of the individual, or because a connection between the two has
proved useful in the history of the race. If a man and his dog stroll
together down the street they turn to the right hand or the left,
hesitate or hurry in crossing the road, recognise and act upon the
bicycle bell and the cabman's shout, by using the same process of
inference to guide the same group of impulses. Their inferences are for
the most part effortless, though sometimes they will both be seen to
pause until they have settled some point by wordless deliberation. It is
only when a decision has to be taken affecting the more distant purposes
of his life that the man enters on a region of definitely rational
thought where the dog cannot follow him, in which he uses words, and is
more or less conscious of his own logical methods.

But the weakness of inference by automatic association as an instrument
of thought consists in the fact that either of a pair of associated
ideas may call up the other without reference to their logical
connection. The effect calls up the cause as freely as the cause calls
up the effect. A patient under a hypnotic trance is wonderfully rapid
and fertile in drawing inferences, but he hunts the scent backward as
easily as