The wise man, indeed, never alters his decision while the conditions under which he formed it remain the same; consequently he never repents, because at the time nothing better than what he did could have been done, and no better decision could have been arrived at than that which was made; yet he begins everything with the saving clause, "If nothing shall occur to the contrary." This is why we say that all goes well with him, and that nothing happens contrary to his expectations, because he forecasts in his mind the possibility of something taking place to prevent his projects being realized. It is the part of an inexperienced man to believe that things will turn out as we wish; the wise man takes account of every possibility.
The reserve clause is not pessimism. It is a hygiene of intention. You go after the thing wholeheartedly; you simply do not stake your equanimity on its arrival. Marcus's version is shorter — "circumstances permitting." Modern translation: do the work without making the outcome a hostage to your peace of mind.
Seneca, De Beneficiis (On Benefits) 4.34 · trans. John W. Basore (1935)
Context
The classical formulation of hupexairesis — the "reserve clause" by which the Stoic forms intentions. Seneca articulates the doctrine across several works (De Tranquillitate Animi 13 carries a parallel statement); this is the fullest form, from De Beneficiis 4.34. You commit to the action; you decline to bind your tranquillity to the outcome.