PHILOSOPHY

(1) the rational investigation of the truths and principles of being, knowledge, or conduct; (2) a system of philosophical doctrine; (3) the study of the basic concepts of a particular branch of knowledge; (4) a system of principles for guidance in practical affairs

Aristotle

Democritus
ancient proponent of the atomic theory

Empedocles

Thales of Miletus

William James
every difference in thinking must make a difference to someone, somewhere

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