Is a good man justified in not feeding his slaves, during a period of great scarcity, in order to save his own children? Let’s assume that in a shipwreck, a foolish man grabs a plank, should a sage be justified in snatching it, in order to save his life, even if it means the sinking of the fool? These two cases question us about how far we are ready to go, in order to save our own lives or the lives of our beloved. By these questions, the Stoics present us with what we can call dilemma: sometimes, whereas the right thing to do is obvious, we can give ourselves very good reasons not to to it; and sometimes, there is no obvious right thing to do, because all the solutions look like bad ones. When it comes to tricky situations, our moral compass seems to blur, which means that our morality is easily shaken. The moral rule, meant to be precisely the inviolable principle of our actions and our reliable criterion, stops begin efficient, when circumstances change. The question raised here is the following:are we justified in making arthmetics, when it comes to moral problems? The Stoics in a way are the first to analyze systematically this very special pattern called “a matter of conscience”, or more broadly “circumstancial duties” (kathekonta kata peristastin), and Stoicism can therefore be seen as the cradle of this new ethical discipline that is casuistry. Coming back to the original scene will be the occasion to understand how the Ancient dealt with such cases, and how they conceived the role of moral rules in daily life.