Epicurus (c. 341–270 BCE) viewed weight as an inherent property of atoms which influences their movement. These atoms move downward in constant free fall within an infinite vacuum without resistance at equal speed, regardless of their mass. On the other hand, upward motion is due to atomic collisions. Epicureans deviated from older atomist theories like Democritus ' (c. 460–c. 370 BCE) by proposing the idea that atoms may randomly deviate from their expected course.