Digital Companion to C. S. Peirce
1902 | Minute Logic: Chapter II. Section II. Why Study Logic? | CP 2.170

If I may be allowed to use the word “habit,” without any implication as to the time or manner in which it took birth, so as to be equivalent to the corrected phrase “habit or disposition,” that is, as some general principle working in a man’s nature to determine how he will act, then an instinct, in the proper sense of the word, is an inherited habit, or in more accurate language, an inherited disposition. But since it is difficult to make sure whether a habit is inherited or is due to infantile training and tradition, I shall ask leave to employ the word “instinct” to cover both cases.

1913 | An Essay toward Improving Our Reasoning in Security and in Uberty | EP 2:464-465

Should I be asked for a more explicit statement of what I mean by an “instinct,” I should [define it], after premissing that while action may, in the first place, be purely physical and open to outward inspection, it may also, in the second place, be purely mental and knowable (by others, at any rate, than the actor) only through outward symptoms or indirect effects, and thirdly it may be partly inward and partly outward, as when a person talks, involving some expenditure of potential energy, - that premissed, I say, I should define what I mean by an “instinct” as a way of voluntary acting prevalent almost universally among otherwise normal individual of at least one sex or other unmistakable natural part of a race (at some stage, or during recurring periods of their lives), which action conduces to the probable perpetuation of that race, and which, in the present stage of science, is not at once satisfactorily and fully explicable as a result of any more general way of mental action.

nd | Miscellaneous Fragments [R] | MS [R] 1573

“Instinct” is nothing but the generalization of a certain kind of single voluntary responses of single animals to single stimuli, namely of those responses that have tendencies to produce similar ultimate (and usually beneficial) effects on the welfare of those animals or of their progeny, which effects, however, they can hardly be supposed to have divined and certainly not to have ascertained by reasoning.