According to Google ngrams the term
willpower wasn't used much before 1880. How did humans reason about motivation back then? What kind of conceptions of human motivation was displaced by adopting the new concept of
According to Google ngrams the term
Scientific psychology only came into being in 19th century, at the end of 18th Kant, now considered one of its forefathers, even opined that psychology can never be a science. Before that it was mostly the domain of philosophers and theologians (much like physics before 17th), Brett's monumental History of Psychology, published in 1912-1921, devotes 90% of space to their writings.
While "willpower" may be recent the "will" as a "power" or "faculty" of the soul is ancient. It is the English translation of Latin "voluntas", which in turn translated Aristotle's "boulesis" (wish, desire). Even before Aristotle Plato in a famous allegory represented human soul as a chariot driven by the two horses, of noble and lowly passions, with the reason for the charioteer. Here is from Kenny's Aquinas on Mind:
"The discussion of the will derives from the philosophical tradition going back to Aristotle. The discussion of free choice harks back to the theological controversy between St. Augustine and his Pelagian opponents concerning the relation between human freedom and divine grace... Aristotle says that the origin of conduct is choice, and the origin of choice is appetition plus means-end reasoning; reasoning by itself moves nothing, only means-end reasoning concerned with good conduct which is the end of appetition. The passage concludes ‘Therefore choice is either appetitive intelligence or ratiocinative appetite’ (Nicomachean Ethics 6, 1139a30ff.)... In the Ethics Aristotle also makes a distinction between wish (boulesis) and choice (prohairesis): wish relates to the end, choice to the means to the end; we wish to be healthy, but we choose what will lead to health...
Aquinas and other scholastics developed this into a full blown theory of soul's cognitive vs appetitive powers. What was common to this tradition, and its 18th century derivatives like Wolffian "rational psychology" that Kant criticized, is the pervasive rationalism about the will and behavior. We were supposed to have a special power to act towards ends dictated by (mostly noble) passions and mediated by rational deliberation about the best means to achieve them. This picture was challenged in 18th century, first by British empiricists (Locke, Berkeley, and most notably Hume), who themselves contributed to what became empirical psychology, and later by German romantics and French vitalists.
Hume, who saw himself as the Newton of psychology, dispensed with the soul itself and its "powers" as fictions, and represented the mental as a bundle of ideas that attract according to the "law of association". The passions he analyzed into "affections", ideas associated to feelings, which sometimes include impulses to act (this is his bundle theory of mind). Hume's mind has no driver, no horses, and is not even a chariot itself, reason and passions are fictions to organize ideas floating around and clustering according to the association laws modeled on Newton's gravity.
While Hume dethroned both the reason and the will romantics and vitalists soon rehabilitated the latter without the former, see Huneman's Montpellier Vitalism and the Emergence of Alienism in France (1750-1800) for a treatment of passions. But it was Schopenhauer's and von Hartmann's irrationalist voluntarism (enabled by Kantian critique of reason, romantic elevation of "animal spirits" by Schelling and others, and the popularity of Oriental philosophies like Vedanta), which paved the way for Freud's semi-empirical psychoanalysis, where deep unconscious urges are placed at the front and center of human motivation. Brett credits the rise of will psychology at the end of 19th century specifically to Schopenhauer's cultural influence, but notes that it was also supported by the early empirical work on psychophysiology of Cabanis and Bichat.