Idealization and Galileo's Proto-Inertial Principle.

Galileo proposed what has been called a proto-inertial principle, according to which a body in horizontal motion will conserve its motion. This statement is only true in counterfactual circumstances where no impediments are present. This article analyzes how Galileo could have been justified in ascribing definite properties to this idealized motion. This analysis is then used to better understand the relation of Galileo's proto-inertial principle to the classical inertial principle. [ABSTRACT FROM AUTHOR]