[W]ith an unquiet mind, neither exercise, nor diet, nor physick can be of much use.

Johnson’s remark links bodily health to the state of the mind: if one is inwardly agitated—anxious, guilty, grief-stricken, or otherwise mentally unsettled—then the usual regimen of health (exercise, diet, and medicine) will have limited effect. The line reflects an 18th‑century moral-psychological view in which passions and worries could undermine physical well-being, and it also fits Johnson’s broader preoccupation with melancholy and the management of the inner life. The emphasis is not that medicine is useless, but that mental tranquility is a necessary condition for its full benefit.