Greek astronomer Hipparchus of Nicaea (c.190 – c. 120 BCE) also rejected Aristotelian physics and followed Strato in adopting some form of theory of impetus to explain motion. The poem De rerum natura by Lucretius (c. 99 – c. 55 BCE) asserts that more massive bodies fall faster in a medium because the latter resists less, but in a vacuum fall with equal speed. Roman engineer and architect Vitruvius (c. 85 – c. 15 BCE) contends in his De architectura that gravity is not dependent on a substance's weight but rather on its 'nature' (cf. specific gravity):