Atomism (from Greek , atomon, i.e. "uncuttable", "indivisible") is a natural philosophy that developed in several ancient traditions. The atomists theorized that nature consists of two fundamental principles: atom and void. Unlike their modern scientific namesake in atomic theory, philosophical atoms come in an infinite variety of shapes and sizes, each indestructible, immutable and surrounded by a void where they collide with the others or hook together forming a cluster. Clusters of different shapes, arrangements, and positions give rise to the various macroscopic substances in the world.