References to the concept of atomism and its atoms appeared in both ancient Greek and ancient Indian philosophical traditions. The ancient Greek 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.