Zeno of Citium (c. 334-262 BCE) was an ancient Greek philosopher and the founder of Stoicism, a school of philosophy that emphasized rationality, virtue, and the development of moral character. Zeno taught that the highest good is living in accordance with nature and reason, and that by cultivating wisdom, courage, justice, and self-discipline, one can achieve inner peace and tranquility, irrespective of external circumstances. He also believed in the universal brotherhood of humanity, asserting that all people share a common rational nature.