The concept of inertia expresses the idea of a kinetic state which changes only if
external actions are applied. Newton’s works is based on the inertia principle, in which it
is established that rest and rectilinear uniform motion are the only kinetic states which do
not change unless an external action intervenes. This conception was not to be given for
granted in the 17th century because many great scientists such as Kepler and Galileo
formulated a different notion of inertia and before the 17th century there was no agreement
on which motion could be considered inertial. Descartes’ first two laws of motion are
verbally similar to the inertia principle, but the development of Cartesian physics makes it
difficult to understand what exactly Descartes meant with his laws. Huygens had, instead,
far clearer ideas on inertia than anyone of his predecessors. The big problems the physicists
had to tackle in the second half of the 17th century was to determine in respect to what a
motion can be regarded as rectlinear and uniform. Here Huygens’ and Newton’s paths
diverge: the latter referred inertia to an absolute space at rest. Huygens refused to ascribe
any kinetic state to the mundane space. His conception of motion was purely relativistic.
In this contribution I will present the strong and the weak aspects of Huygens’ ideas on
inertia in the light of some recent researches.
