SENECA; Davie, John; Reinhardt, Tobias (2007-10-11). Dialogues and Essays (Oxford World’s Classics) . Oxford University Press. Kindle Edition.

In some respects, Stoicism can be seen as a systematized version of views which can be drawn from the argumentative positions Socrates adopts in the various dialogues of Plato. At its heart lies the notion that the only thing in life that actually matters and is worth caring about is the self, that is, the soul; that whether one has a good life or not crucially depends only on factors which affect the soul; and that in order to have a good life we need wisdom, that is, a certain kind of knowledge of what is good and bad. For the Stoics, this kind of knowledge is virtue, and the various virtues the Greeks traditionally distinguished are aspects of that knowledge. Given the importance accorded to care for the self, the Stoics treated most of the things that ordinary people either desire or dread in life as ‘indifferents’ (adiaphora), but made a distinction between ‘preferred indifferents’, which are ‘in accordance with nature’, and others (see below on the conception of nature at issue here, p. xiii). Thus, while it is preferable to be healthy, not in material need, and to enjoy social prestige, all of these things are external to the good life, in that they do not affect the soul, so that not obtaining them does not make a life bad; likewise, suffering great pain or misfortune, or having one’s life cut short in the bloom of youth, while not to be preferred, do nothing to make a life bad; indeed, within a broader context, which places us within the world as a whole, there may even be a sense in which our life is enhanced by such occurrences. Virtue alone is the good for the Stoics, and sufficient for happiness. This is the most extreme conception of virtue to be found in antiquity. To be virtuous means to be perfectly rational and to know both how to act in private life and with respect to one’s friends, business associates, fellow citizens or countrymen, or indeed other members of the human race.”