Who said, "What goes up must come down"?

The quote explains gravity that was studied by Isaac Newton.

Sir Isaac Newton was an English mathematician, physicist, astronomer, theologian, and author (described in his own day as a "natural philosopher") who is widely recognized as one of the most influential scientists of all time, and a key figure in the scientific revolution. His book "Philosophiæ Naturalis Principia Mathematica", first published in 1687, laid the foundations of classical mechanics. Newton also made seminal contributions to optics and shares credit with Gottfried Wilhelm Leibniz for developing the infinitesimal calculus.

Newton presented a calculus-like method of geometrical analysis using 'first and last ratios', gave the first analytical determination (based on Boyle's law) of the speed of sound in air, inferred the oblateness of Earth's spheroidal figure, accounted for the precession of the equinoxes as a result of the Moon's gravitational attraction on the Earth's oblateness, initiated the gravitational study of the irregularities in the motion of the Moon, provided a theory for the determination of the orbits of comets, and much more.