A man in health possesses more of it than one languishing with sickness. We are more master of our thoughts in the morning than in the evening
 David Hume, An Enquiry Concerning Human Understanding (1748). copy citation

Context

“Will any one pretend to assign the ultimate reason of these boundaries, or show why the power is deficient in one case, not in another. Thirdly, This self-command is very different at different times. A man in health possesses more of it than one languishing with sickness. We are more master of our thoughts in the morning than in the evening: Fasting, than after a full meal. Can we give any reason for these variations, except experience? Where then is the power, of which we pretend to be conscious? Is there not here, either in a spiritual or material substance, or both, some secret mechanism or structure of parts, upon which the effect depends, and which, being entirely unknown to us, renders the power or energy of the will equally unknown and incomprehensible?” source