The problem of explaining how certain properties such as hotness or color can vary in intensity was one of the most debated issues in late medieval philosophy, and led to some of the most original innovations of 14th-century physics.

The dominant Aristotelian framework of natural philosophy in the 14th century left no room for explaining intensive properties in terms of corpuscular motions and, generally speaking, in mechanical terms. It is usually assumed that among the competing theories designed to solve the problem, one of them – the “addition theory” – eventually won the battle and was generally adopted in the 14th century, paving the way for a quantification of natural phenomena that eventually led to modern physics.

In this talk, I would like to challenge this view and show that some of the most influential thinkers of mid-fourteenth century, namely John Buridan and Marsilius of Inghen, defended an alternative theory. After recalling the initial debate and the major theories of intensive properties, it will be shown that the positions defended by Buridan and Marsilius revert to a particular theory – the “admixture theory” – that was largely rejected in the first stages of the debate. I will show that this theory could only be defended in a new formulation, owing to a modification of an important Aristotelian axiom, which had strong implications for the evolution of late medieval physics.