INTRODUCTION. 15 
lion, and the rest, as implying mental faculties the same 
in kind as those which in ourselves we call rational. 
Now it is notorious that no distinct line can be drawn 
between instinct and reason. Whether we look to the 
growing child or to the ascending scale of animal life, we 
find that instinct shades into reason by imperceptible 
degrees, or, as Pope expresses it, that these principles are 
6 for ever separate, yet for ever near . 5 Nor is this other 
than the principles of evolution would lead us to expect, 
as I shall afterwards have abundant occasion to show. 
Here, however, we are only concerned with drawing what 
distinction we can between instinct and reason as these 
faculties are actually presented to our observation. And 
this in a general way it is not difficult to do. 
We have seen that instinct involves * mental opera- 
tions , 5 and that by this feature it is distinguished from re- 
flex action ; we have now to consider the features by which 
it is distinguished from reason. These are accurately, 
though not completely, conveyed by Sir Benjamin Brodie, 
who defines instinct as 6 a principle by which animals 
are induced, independently of experience and reason- 
ing, to the performances of certain voluntary acts, which 
are necessary to their preservation as individuals, or 
to the continuance of the species, or in some other 
way convenient to them . 5 1 This definition, as I have 
said, is accurate as far as it goes, but it does not state 
with sufficient generality and terseness that all instinctive 
action is adaptive ; nor does it clearly bring out the dis- 
tinction between instinct and reason which is thus well 
conveyed by the definition of Hartmann, who says in his 
6 Philosophy of the Unconscious , 5 that 4 instinct is action 
taken in pursuance of an end, but without conscious per- 
ception of what the end is . 5 This definition, however, is 
likewise defective in that it omits another of the im- 
portant differentiae of instinct- — namely, the uniformity of 
instinctive action as performed by different individuals of 
[he same species. Including this feature, therefore, we 
may more accurately and completely define instinct as 
mental action (whether in animals or human beings), 
1 Psychological Researches , p. 187, 
3 
