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.