Macaulay might even further have admitted that the mental act of calculation itself results from, or is accompanied by, an impulse to calculate, which impulse may have nothing to do with any anterior consideration of means and ends, and may vary from the half-conscious yielding to a train of reverie up to the obstinate driving of a tired brain onto the difficult task of exact thought.

The text-books of psychology now warn every student against the ‘intellectualist’ fallacy which is illustrated by my quotation from Macaulay.  Impulse, it is now agreed, has an evolutionary history of its own earlier than the history of those intellectual processes by which it is often directed and modified.  Our inherited organisation inclines us to re-act in certain ways to certain stimuli because such reactions have been useful in the past in preserving our species.  Some of the reactions are what we call specifically ‘instincts,’ that is to say, impulses towards definite acts or series of acts, independent of any conscious anticipation of their probable effects.[4] Those instincts are sometimes unconscious and involuntary; and sometimes, in the case of ourselves and apparently of other higher animals, they are conscious and voluntary.  But the connection between means and ends which they exhibit is the result not of any contrivance by the actor, but of the survival, in the past, of the ‘fittest’ of many varying tendencies to act.  Indeed the instinct persists when it is obviously useless, as in the case of a dog who turns round to flatten the grass before lying down on a carpet; and even when it is known to be dangerous, as when a man recovering from typhoid hungers for solid food.

[4] ’Instinct is usually defined as the faculty of acting in such a way as to produce certain ends without foresight of the ends and without previous education in the performance.’—­W.  James, Principles of Psychology, vol. ii. p. 383.

The fact that impulse is not always the result of conscious foresight is most clearly seen in the case of children.  The first impulses of a baby to suck, or to grasp, are obviously ‘instinctive.’  But even when the unconscious or unremembered condition of infancy has been succeeded by the connected consciousness of childhood, the child will fly to his mother and hide his face in her skirts when he sees a harmless stranger.  Later on he will torture small beasts and run away from big beasts, or steal fruit, or climb trees, though no one has suggested such actions to him, and though he may expect disagreeable results from them.

We generally think of ‘instinct’ as consisting of a number of such separate tendencies, each towards some distinct act or series of acts.  But there is no reason to suppose that the whole body of inherited impulse even among non-human animals has ever been divisible in that way.  The evolutionary history of impulse must have been very complicated.  An impulse which survived because it produced one result