(3) Finally there seem to have been some anticipa-
tions during the latter Middle Ages of modern theories
of force and mass. Jean Buridan (ca. 1299-ca. 1358),
from whose rejection of Intelligences as movers of the
heavenly spheres Duhem dated the beginning of mod-
ern science (Etudes sur Léonard de Vinci [1955], III,
ix) helped to develop a theory of the preservation of
motion by an originally impressed impetus which acted
without diminution so long as it met no resistance. As
for mass, Jammer finds significant the use by Giles of
Rome, in his Theoremata de corpore Christi (1276), of
the phrase Quantitas materiae in a meaning exclusive
of both volume and weight. Prior to the conceptualiza-
tion of inertial mass by Kepler, natural philosophers
like Buridan, Albert of Saxony, Nicole Oresme, and
Richard Swineshead were identifying quantity of mat-
ter with a product of volume times density. In spite
of the greater specificity with which questions about
matter were being put, thanks to awakened interest,
suspicion of traditional answers, and improved tech-
niques, the late medievals had, of course, no way of
determining quantity of matter and density inde-
pendently.
