The Stoic philosophy developed from that of the Cynics whose founder, Antisthenes, had been a disciple of Socrates. The Stoics emphasized ethics as the main field of knowledge, but they also developed theories of logic and natural science to support their ethical doctrines.
Holding a somewhat materialistic conception of nature they followed Heraclitus in believing the primary substance to be fire. They also embraced his concept of Logos which they identified with the energy, law, reason, and providence found throughout nature.
They held Logos to be the animating or 'active principle' of all reality. The Logos was conceived as a rational divine power that orders and directs the universe; it was identified with God, nature, and fate. Human reason and the human soul were both considered part of the divine Logos, and therefore immortal.
The foundation of Stoic ethics is the principle, proclaimed earlier by the Cynics, that good lies in the state of the soul itself, in wisdom and restraint. Stoic ethics stressed the rule "Follow where Reason leads"; one must therefore resist the influence of the passions-love, hate, fear, pain, and pleasure.
Living according to nature or reason, they held, is living in conformity with the divine order of the universe. The four cardinal virtues of the Stoic philosophy are wisdom, courage, justice, and temperance, a classification derived from the teachings of Plato.
A distinctive feature of Stoicism is its cosmopolitanism. All people are manifestations of the one universal spirit and should, according to the Stoics, live in brotherly love and readily help one another. They held that external differences such as rank and wealth are of no importance in social relationships. Thus, before the rise of Christianity, Stoics recognized and advocated the brotherhood of humanity and the natural equality of all human beings. Stoicism became the most influential school of the Greco-Roman world and produced a number of remarkable writers and personalities.