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