The notion of mental reserves comes from William James's 1907 essay on The Energies of Men. Just like there are physical reserves that are released when we exceed some initial physical effort, James argues that there are mental reserves that can be released if we keep pushing past our initial mental effort.
He further goes on to say that we all live at a particular vital equilibrium but "at very different rates of energizing,
" and that only a "few men live at their maximum of energy.
" If true, this has drastic implications for our life, "a man who energizes below his normal maximum fails by just so much to profit by his chance at life.
Every one is familiar with the phenomenon of feeling more or less alive on different days. Every one knows on any given day that there are energies slumbering in him which the incitements of that day do not call forth, but which he might display if these were greater. Most of us feel as if a sort of cloud weighed upon us, keeping us below our highest notch of clearness in discernment, sureness in reasoning, or firmness in deciding. Compared with what we ought to be, we are only half awake. Our fires are damped, our drafts are checked. We are making use of only a small part of our possible mental and physical resources.
It seems clear that James is here referring to an extraordinary, overall elevation and clarity of mind, rather than a plain industrious intelligence. If James is right, we can get to this state by tapping into our mental reserves, and we can stay in that state indefinitely if we are able to establish a new vital equilibrium that becomes our norm. In other words, if we create habits that require more from us, at least up to a certain point, we will produce more energy to meet that need with no adverse side-effects.
The notion of "mental reserves" has gone out of fashion in psychology, but it is generally recognized that one of the main bottle-necks of mental activity is energy. This is because the cells in our body only have a certain short term storage of energy for "explosive" use. If we strain ourselves physically (think pull-ups), it takes a certain time for the cells to re-charge. The same principle constrains neuronal cells when we think, particularly for operations such as working memory.
James also hold that the notion of "mental reserves" can apply to mental abilities such as willpower. According to recent (pragmatic) studies in psychology on willpower, each choice we make - in proportion to its demand from us - will exhaust a certain amount of willpower that we only replenish after rest or sleep. If James' theory applies here, we can likely exercise and reach new levels of willpower as well.
In my own experience my mental abilities (willpower, creativity, honesty, empathy, etc.) all seem to vary incredibly from time to time. The prospect that it is possible to sustain those rare peaks simply by putting in more effort is mind-blowing, and makes me optimistic that personal growth is possible beyond the incremental and mundane.