René Descartes (1596-1650)
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The one who said no to the void. Where the atomists split the world into atoms and empty space, Descartes denied that empty space could exist at all: matter went all the way down, packed without gaps, and motion happened as swirling vortices in a completely full universe — a plenum. He is the great opponent in this story, the mechanical philosophy without the void. He also handed mathematics a tool it still uses every day: analytic geometry, algebra and geometry fused onto a coordinate grid — the "Cartesian" plane.
Plenum versus void:
Leucippus invented atoms-plus-void to let change happen: things move because there is empty space to move into. Descartes refused the empty space. For him extension just is matter, so a truly empty space is a contradiction — the cosmos is packed solid, and motion is matter shoving matter in great whirlpools (vortices), which is how he explained the orbits of the planets. The void-versus-plenum argument ran for a century; Torricelli’s barometer and Newton’s mechanics eventually decided it for the void — but Descartes forced the atomists to actually defend the emptiness their whole picture depended on.