How did Democritus try to reconcile unity in nature with the many forms people observe?

Democritus reconciled the unity of nature with the observable multiplicity of forms by distinguishing between reality and convention. He argued that the only things that exist in truth are atoms and the void.1

To explain the diverse qualities people observe—such as color, taste, or hardness—Democritus proposed that these are not inherent properties of matter but are merely conventional impressions caused by the configuration, arrangement, and motion of atoms. While the atoms themselves are eternal, indivisible, and homogeneous, they differ in purely quantitative ways: size, shape, and position.12

For example, he suggested that sensations like sweetness or bitterness result from the specific size and shape of atoms impinging on the soul, while the physical state of a substance (such as liquid water versus solid iron) depends on whether the atoms are smooth enough to roll over one another or jagged enough to cling together. By reducing all qualitative differences to these quantitative arrangements, Democritus maintained a fundamental unity in nature, asserting that nothing truly comes into being or perishes; rather, only the combinations of these unchanging atoms shift over time.12