Equanimity made a dramatic appearance in the field of medicine in the late nineteenth century. In his 1889 address to medical graduates at the University of Pennsylvania which also served as his farewell lecture before he departed to become chief of staff at Johns Hopkins Hospital, the Canadian physician Sir William Osler (1849–1919), FRS, FRCP, advised his students to understand the relationship of imperturbability with equanimity. Drawing upon the philosophical ideal of aequanimitas, he stressed that their future careers as physicians depended in no small part upon ‘coolness and presence of mind under all circumstances, calmness amid storm, clearness of judgment in moments of grave peril’.^{Reference Osler1} As the ‘mental equivalent’^{Reference Osler1} to imperturbability, a concept, he pointed out, involving ‘immobility, impassiveness, or, to use an old and expressive word, phlegm’,^{Reference Osler1} equanimity involves ‘educat[ing] your nerve centers’^{Reference Osler1} in order to find a balance between constriction and expansion. What Osler, one of modern medicine's most influential and admired physicians, means by this is not the concealment of empathy but rather an open attitude of equilibrium that patients find calming, reassuring.

Equanimity has deeps roots in Western sources (e.g. the Stoic philosophy upon which Osler explicitly drew) and Eastern ones, especially in Buddhist and Taoist traditions. For example, in Buddhism, there is a wonderful synonym for upekṣā (Sanskrit) – or upekkha (Pali) – the term most commonly translated as equanimity. The word is tatramajjhattatā, a compound of three Pali elements meaning standing there in the middle of everything. (Pali is the language of the Tipiṭaka, the sacred canon of Theravāda Buddhism.) This middle position is a privileged one. Here, because experience is not split into the categories of pleasant or unpleasant, immunity to immersion and avoidance may be found.

The awakened mind in the middle possesses a broad state of awareness, and is like an undivided context for the experience of things. In the ‘The Parable of the Plants’ of the Lotus Sūtra, it is compared to falling rain:

Never reducible to a state of stillness, true equanimity has the conditions of vibrancy and range characterising the awakened mind. As an image of how consciousness operates generatively, it is like falling rain – it ‘goes to all’. Equanimity is thus the function of a flexible perceptual system, ranging over whatever may appear and fluidly noticing both the pleasant and the unpleasant with an awareness that one depends on the other for its meaning. Through an orientation toward the fullness of what is, equanimity is most recognisable.