Thales of Miletus (c. 625-547 BC), a pre-Socratic philosopher, is regarded as the father of Greek philosophy, and the first Greek scientist and mathematician (although he was an engineer by training). He belived that natural phenomena could be explained by laws rather than resorting to supernatural explanations.

This means that water is the one primal kind of existence and that everything else in the universe is merely a modification of water. In Aristotle’s terminology, Thales maintained that water is the material cause of all things.

Thales was the first to discover the period of one solstice to the next. He discovered the seasons, which he divided into 365 days. He was the first to state that the size of the Sun was 1/720 part of the solar orbit just as the Moon was 1/720 part of the lunar orbit.

Aristotle

René Descartes

The separation of philosophy and science from theology began in Greece during the 6th century BC. Thales, an astronomer and mathematician, was considered by Aristotle to be the first philosopher of the Greek tradition. While Pythagoras coined the word, the first known elaboration on the topic was conducted by Plato.

René Descartes is generally considered the father of modern philosophy. He was the first major figure in the philosophical movement known as rationalism, a method of understanding the world based on the use of reason as the means to attain knowledge.

Descartes’s influence in mathematics is equally apparent; the Cartesian coordinate system was named after him. He is credited as the father of analytical geometry, the bridge between algebra and geometry—used in the discovery of infinitesimal calculus and analysis.

Gabriel Marcel

Essentialism is the view that certain categories (e.g., women, racial groups, dinosaurs, original Picasso artwork) have an underlying reality or true nature that one cannot observe directly.