In the paper I show why we should consider Stoicism as the historical source of St. Thomas’s distinction between ‘conscience’ and ‘synderesis’. I claim that the Stoic terms syntērēsis and syneidēsis became, through the ages, the Thomistic synderesis and conscientia. The Stoic syntērēsis meant ‘self-preservation’, and in all animals this ‘first instinct’ refers to the body. The man is the only creature, which, because of its ‘rational nature’, preserves not necessarily its body but rather its soul, i.e. a system of values. Such preservation of someone’s axiological integrity equals ‘salvation’, and thus assimilates Stoicism to Christianity. In the Stoic system, human values follow ‘the nature’ (or ‘the human nature’ in particular), and in Thomism, they follow ‘synderesis’, or the natural inclination toward the good. In both cases we find a natural instinct that transforms itself into a rational structure of conscience. I also argue that, thanks to the moral phenomena of ‘adaptation’ (oikeiōsis) and ‘advancement’ (teleiōsis), the Stoic ethics is not completely egocentric, but incorporates also social duties.