Close and severe thinking

Slight desultory contemplations leave transient impressions, but deep continued study exhausts both body and mind. It has been supposed, and perhaps justly, that all our thoughts, sensations, and intellectual exertions, are accompanied with correspondent motions; close and severe thinking has a direct tendency to weaken, confuse, and destroy the intellect.

C***
Practical Observations on Insanity, p. 31.