We know about atoms because of scientists and electron microscopes, but the idea goes back to the ancient Greeks.
Democritus first articulated the idea of atoms. He argued that it must exist. If we continually halved something, there would be no endpoint. Therefore, there must be a fundamental unit to the world from which everything else is made.
One of Democritus' question was how atoms made objects. He noticed that the world is constantly changing, dying, growing. The worlds flow, therefore atoms must flow. They can't just be still.
Democritus reasoned that atoms join into various combinations, then emit eidôla (composite blobs of atoms) outward, like ripples in water. This atomic radiation is then picked up by us and translated into ideas or sensations.
Democritus was then the first "determinist" in that he stated that there could be no free will or choice - we are just moving to the laws of physics. His conclusions about atoms could only be proven millennia later.
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