If I hold a twenty pound weight, I cannot detect a fly landing on it because the least detectable difference in the stimulus is half a pound. On the other hand, if i hold a feather, a fly landing on it makes a great difference. Obviously then, in order to be able to tell the differences in exertion one must first reduce the exertion. Finer and finer performance is possible only if the sensitivity, that is, the ability to feel the difference is improved.
