The Greek word behind reason also meant an account of money
Logos, the ancient Greek root of logic, could mean speech, explanation, or a financial account. Latin translated it as ratio, which also served for balancing the books. From that bookkeeping ancestry came one of philosophy's grandest ideas: that reason sets humans apart and even links them to the order of the cosmos.
Reason is the capacity to apply logic deliberately, drawing valid conclusions from what we know in pursuit of truth. It is tied to activities seen as distinctly human, such as science, mathematics and language, and splits into deductive, inductive and abductive forms. Aristotle separated step-by-step reasoning from intuition, which can be valid but personal and opaque; in mathematics the two cooperate, since intuition often leads the way to a formal proof. The English word arrived via French raison from Latin, and early English philosophers such as Hobbes and Locke treated logos, ratio, raison and reason as interchangeable.
Many classical thinkers believed the cosmos itself was rational, perhaps from Pythagoras or Heraclitus onward, so human reason connected an immortal part of the mind to a divine order. Plato described reason as the rightful ruler of the soul, governing spirit and appetite. Aristotle defined humans as rational animals and said the happiest life is one lived fully in line with reason. Plotinus imagined a single world soul as the seat of all reason, and early Church Fathers like Augustine absorbed that Neoplatonic picture.
Medieval thought carried it further. Islamic philosophers including Avicenna and Averroes shaped the Scholastic view, and Thomas Aquinas made reason central to his natural law, arguing that because reason is a spark of the divine, every human life is precious and all people share basic rights, a foundation later built on by the School of Salamanca in developing the idea of human rights. Roger Bacon and Albertus Magnus, following the Islamic scholar Alhazen, treated reason as a tool for decoding nature, which helped the scientific method take shape in early universities.
The early modern era broke that frame. Nature stopped being seen as purposeful, and Descartes rejected the idea of humans as rational animals, calling them thinking things and doubting everything except the thinking mind. Hobbes cast reason as a kind of generalised addition and subtraction. David Hume pushed hardest, arguing that cause and effect cannot be deduced, so no knowledge rests on reasoning alone.
Source: Reason
