In his third letter to Lucilius, Seneca urges his friend to consider that the Stoic sage would be fearless, for the only thing that could truly harm him would be to choose to act contrary to Virtue (which the sage, of course, would not do). A man of perfect reason would not fear anything . . . But of course, few to none of us are sages, few to none of us have the perfect reason of a sage, and so therefore . . . we fear . . .
"Adhuc enim non pueritia sed, quod est gravius, puerilitas remanet. Et hoc quidem peior est, quod auctoritatem habemus senum, vitia puerorum, nec puerorum tanatum sed infantum. Illi levia, hi falsa formidant, nos utraque."
"It is not childhood [lit. "boyhood"] that still stays with us, but what is more serious, childishness [lit. "boyishness"]. And this is indeed worse, because we have the authority of adults [lit. "of old men"], the failings of children [lit. "boys"], nay, not even children, but of infants. Children [lit. "boys"] fear trifles, infants fear shadows [lit. "false things"], we [adults] fear both."
Seneca's point seems to be that we are born without much power of reason, and so as infants we fear everything, and many of the things we fear do not even exist (falsa). As we grow, we develop our power of reason, but as children, we still have much to learn about how to apply our faculties, so we continue to fear trifles. But as adults, with fully developed powers of reason, and having had plenty of opportunities to learn to apply our faculties, we ought to be fearless . . . in a perfect world. However, since few to none of us is endowed with perfect reason (as the mythic Stoic sage), even as adults, most of us continue to fear trifles and shadows we should have long since outgrown. For what is there to fear, as long as the choice of good and evil is ours to make? Choose Virtue, shun Vice, and there is literally nothing to fear.