The problems of fatigue have recently, for well-known reasons, attracted unusual attention. They are, however, ancient problems around which a bewildering and contradictory mass of work and literature has accumulated. The author, who has for many years been working on the subject, here deals (in a Presidential Address to the American Psychological Association last year) with one limited aspect of these problems: the relativity of fatigue. He is more concerned with the scientific than with the practical aspects of the subject, for he considers that the extreme practical importance of fatigue has injured its proper scientific investigation. For this we must know what mental fatigue is, if it exists at all, and how it is conditioned.