The term ‘heat’ originates from the Old English word hǣtu, a word of Germanic origin; related to the Dutch ‘hitte’ and German ‘Hitze’. Today, we distinguish three different meanings of the word ‘heat’. First, ‘heat’ is understood in colloquial English as ‘hotness’. There are, in addition, two scientific meanings of ‘heat’. ‘Heat’ can have the meaning of the portion of energy that changes with a change of temperature. And finally, ‘heat’ can have the meaning of the transfer of thermal energy from a hotter to a colder system or body.
By contrast, for the Ancients and Scholastics, ‘heat’ was a manifest, real quality of bodies and there was an ontological distinction between biological or innate heat (which was regarded as an innate principle of life for warm-blooded animals) and the physical manifest heat of external objects, which is potentially harmful. During the late Renaissance period, however, both views changed fundamentally and evolved - via the application of physical and mechanical analogies - into the foundations for today’s unified mechanistic theory of heat.