I read the quote as remarking on the problem of implementation—people often can enunciate the optimal course of action for themselves in their present situation (e.g. I should be working on my paper right now) without this enunciation having the slightest effect on their behavior. Therefore, since the benefits of rationality only accrue to those whose behavior is rational, no art of rationality is complete that does not deal with implementation.
I read the quote as remarking on the problem of implementation—people often can enunciate the optimal course of action for themselves in their present situation (e.g. I should be working on my paper right now) without this enunciation having the slightest effect on their behavior. Therefore, since the benefits of rationality only accrue to those whose behavior is rational, no art of rationality is complete that does not deal with implementation.