Feelings of fatigue can be found in association with a number of physical and psychiatric conditions, and form part of the experiential meaning of technical and vulgar terms such as asthenia, anergy, tiredness, weariness, languor, lassitude, depression, melancholia, acedia, apathy, inertia, aboulia, lethargy, exhaustion, vecordia, tediousness, ennui, debility, lack of vitality, lack of vigour, pusillanimity, adynamia, boredom, feebleness, failing of strength, hyperaesthesis, irritability, anhedonia, ‘being out of sorts’, ‘out of being’, ‘feeling knackered’, and ‘down in the dumps’.

Before embarking on the history of these phenomena, the semantic field of ‘feelings of fatigue’ and cognate terms must be clarified; to do so five questions may be asked. The first is whether such terms have a common denominator; it would seem that they do partake in a ‘primary sensation’ conferring upon them a sort of ‘family resemblance’. The second question is whether such common sensation is distinctive enough to be differentiated from others such as mild pain; it will be shown that feelings of fatigue have been historically considered as a primary experience. The third question is whether it is distinctive enough to be recognized even when not accompanied by its usual causal associations, e.g. when experienced in situations not preceded by exertion; clinical observation shows that subjects respond with alarm to the presence of feeling of fatigue when it appears ‘unexplained’. The fourth question is whether this unexplained feeling of fatigue is phenomenologically identical to that experienced after exercise; the answer to this question must be sought in empirical research.