Aristotle
Notes
Aristotle (384–322 BC), Greek philosopher, student of Plato, tutor of Alexander the Great and founder of the Lyceum, where Theophrastus succeeded him. He wrote on logic, physics, biology, ethics, politics, rhetoric and poetry, invented formal logic more or less single-handedly, and was the first philosopher to insist that enquiry start from observation rather than from the Forms. For roughly two thousand years "the Philosopher" meant Aristotle.
He appears here in three ways. His unmoved mover is the ancestor of every cosmological argument, including the Kalam Cosmological Argument and the First Cause Argument, and Thomas Aquinas built the Five Ways on him. His virtue ethics, in which the good life is one of flourishing (eudaimonia) rather than of rule-following, is the sanest classical account of Happiness. And his catalogue of fallacies in the Sophistical Refutations is the origin of the fallacy notes in Thinking Tools; Equivocation and the Composition Fallacy are his.
Wikipedia: https://en.wikipedia.org/wiki/Aristotle