In The Philosophy of Cognitive-Behavioural Therapy, Donald Robertson discusses what he calls the "reserve clause" (exceptio) that constitutes "one of the most basic underlying concepts of Stoicism." Robertson sees it as a formulation from a different perspective of what he calls the "Stoic fork" - the approach to all things with the realization that there are some things that are under our control (anything having to do with moral choice) and some things that are not under our control (all external things). The "reserve clause" is "a verbal clause added to the end of each sentence concerning one's own actions or intentions. Or rather, it is the concept which would be implied by adding such a clause, the idea that it expresses. because . . . the Stoic went from learning to merely say the reserve clause to actually experiencing it. The clause itself can take several forms, for example, 'God willing', 'fate willing', 'nature permitting', 'if nothing prevents me', etc. In each case, however, the underlying idea is essentially the same. A common proverb expresses it thus: 'Do what you must; let happen what may'." Robertson goes on to point out that Seneca says that "the Stoic sage undertakes every action with the reserve clause: 'If nothing shall occur to the contrary' (Seneca, 2009, p. 116)."
Robertson concludes that "the Stoic, therefore, makes a point of qualifying the expression of every intention by introducing a distinction between his will and external factors beyond his control. The sage, thereby, holds two complementary propositions in mind simultaneously, viz.,
1. I will do my very best to succeed . . .
2. while simultaneously accepting that the ultimate outcome is beyond my direct control."