Butter undergoing phase change in a hot skillet due to conductive heat transfer from a hot electric stove, through a layer of cast iron, and through the boundary of the skillet; image from J. M. Powers, January 2018.

Primary causes are unknown to us; but are subject to simple and constant laws, which may be discovered by observation, the study of them being the object of natural philosophy. Heat, like gravity, penetrates every substance of the universe, its rays occupy all parts of space. The object of our work is to set forth the mathematical laws which this element obeys. The theory of heat will hereafter form one of the most important branches of general physics.

Jean-Baptiste Joseph Fourier, 1768-1830, mathematician and thermal physicist