I think this notion parallels modern notions of arousal and alertness because Crichton felt that attention could become fatigued or be affected by inadequate mental energy.
The first paper in the medical literature on disorders of attention such as ADHD is a short chapter on this topic in a medical textbook (published anonymously, initially) by Melchior Adam Weikard in 1775 (Barkley & Peters, 2012).
Here again we see reference to the possibility that ADHD may persist into adulthood thereby logically opening the door to the possibility that adults can possess this same pattern of symptoms dating back to childhood.
The first papers on research studies involving adults having actual ADHD seem to date to the late 1960s.
Most of his cases were not just inattentive and lacking in forethought but also were quite overactive.
He proposed that the immediate gratification of the self was the “keynote” quality of these and other attributes of the children.