Sir Isaac Newton FRS (4 January 1643 – 31 March 1727 [OS: 25 December 1642 – 20 March 1727]) was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, and is considered by many scholars and members of the general public to be one of the most influential people in human history. His Philosophiæ Naturalis Principia Mathematica (Latin for "Mathematical Principles of Natural Philosophy"; usually called the Principia), published in 1687, is probably the most important scientific book ever written. It lays the groundwork for most of classical mechanics. In this work, Newton described universal gravitation and the three laws of motion, which dominated the scientific view of the physical universe for the next three centuries. Newton showed that the motions of objects on Earth and of celestial bodies are governed by the same set of natural laws, by demonstrating the consistency between Kepler's laws of planetary motion and his theory of gravitation; thus removing the last doubts about heliocentrism and advancing the Scientific Revolution.

Newton built the first practical reflecting telescope and developed a theory of colour based on the observation that a prism decomposes white light into the many colours that form the visible spectrum. He also formulated an empirical law of cooling and studied the speed of sound.

In mathematics, Newton shares the credit with Gottfried Leibniz for the development of differential and integral calculus. He also demonstrated the generalised binomial theorem, developed Newton's method for approximating the roots of a function, and contributed to the study of power series.

Newton was also highly religious. He was an unorthodox Christian, and during his lifetime actually wrote more on Biblical hermeneutics and occult studies than on science and mathematics, the subjects he is mainly associated with.

- To every reaction there is always opposed an equal reaction.
- To every action there is always opposed an equal reaction.