The evolution and diversity of the world are explained by primæval matter, although uniform and indivisible, consisting of three different substances called gunas or constituents (originally "strands" of a rope). By the combination of these in varying proportions the diverse material products were supposed to have arisen. The constituent, called sattva, distinguished by the qualities of luminousness and lightness in the object, and by virtue, benevolence, and other pleasing attributes in the subject, is associated with the feeling of joy; rajas, distinguished by activity and various hurtful qualities, is associated with pain; and tamas, distinguished by heaviness, rigidity, and darkness on the one hand, and fear, unconsciousness, and so forth, on the other, is associated with apathy. At the end of a cosmic period all things are supposed to be dissolved into primitive matter, the alternations of evolution, existence, and dissolution having neither beginning nor end. The psychology of the Sankhya system is specially important. Peculiarly interesting is its doctrine that all mental operations, such as perception, thinking, willing, are not performed by the soul, but are merely mechanical processes of the internal organs, that is to say, of matter. The soul itself possesses no attributes or qualities, and can only be described negatively. There being no qualitative difference between souls, the principle of personality and identity is supplied by the subtile or internal body, which, chiefly formed of the inner organs and the senses, surrounds and is made conscious by the soul. This internal body, being the vehicle of merit and demerit, which are the basis of transmigration, accompanies the soul on its wanderings from one gross body to another, whether the latter be that of a god, a man, an animal, or a tree. Conscious life is bondage to pain, in which pleasure is included by this peculiarly pessimistic system. When salvation, which is the absolute cessation of pain, is obtained, the internal body is dissolved into its material elements, and the soul, becoming finally isolated, continues to exist individually, but in absolute unconsciousness. The name of the system, which only begins to be mentioned in the later Upanishads, and more frequently in the Mahabharata, is derived from samkhya, "number." There is, however, some doubt as to whether it originally meant "enumeration," from the twenty-five tattvas or principles which it sets forth, or "inferential or discriminative" doctrine, from the method which it pursues.