Democritus said reality is just indivisible atoms moving through empty space
Democritus argued that everything in the universe reduces to indivisible, indestructible atoms moving through empty void, while qualities like colour and taste exist only "by convention," not in the atoms themselves. None of his original writings survive intact; his theory comes down to us mainly through Aristotle's criticism and later doxography. It's often called the first fully materialist account of physical reality in Western philosophy.