 The Jainas also propound atomism. All the entities admitted in their ontology, except souls and space, are constituted by material elements (pudgalas). Atoms are eternal as regards their substance and each exists by occupying one space-point (pradeśa). These atoms are qualitatively similar, each possessing one kind of taste, smell and colour and two kinds of touch, viz. hot or cold and rough or smooth. Other kinds of touch, viz. heavy, light, soft and hard, and varied colour, taste and smell are found only in compounds formed by atoms. The Jainas maintain that atoms are usually in motion but not always. Depending upon the spatio-temporal conditions, atomic motion is either regular (niyamita) or irregular. In one unit of time atoms regularly move in a straight line. However, while in interaction with another atom or a group of atoms, atomic motion becomes curvilinear. The Jainas also speculate about the speed of a moving atom under different conditions.