Chapter 6, Footnote #05
The most accurate measure that can be given of a man's strength, seems to be that which is taken from the weight or number of pounds and ounces he can lift with his hands in a given attitude. This indeed relates immediately only to his arms: but these are the organs of strength which are most employed; of which the strength corresponds with most exactness to the general state of the body with regard to strength; and in which the quantum of strength is easiest measured. Strength may accordingly be distinguished into general and particular.
Weakness is a negative term, and imports the absence of strength. It is, besides, a relative term, and accordingly imports the absence of such a quantity of strength as makes the share, possessed by the person in question, less than that of some person he is compared to. Weakness, when it is at such a degree as to make it painful for a man to perform the motions necessary to the going through the ordinary functions of life, such as to get up, to walk, to dress one's self, and so forth, brings the circumstance of health into question, and puts a man into that sort of condition in which he is said to be in ill health.