The tendency to evolution inherent in matter is due to the three gunas.  They are sattva, explained as goodness and happiness; rajas, as passion and movement; and tamas, as darkness, heaviness and ignorance.  The word Guna is not easy to translate, for it seems to mean more than quality or mode and to signify the constituents of matter.  Hence one cannot help feeling that the whole theory is an attempt to explain the unity and diversity of matter by a phrase, but all Hinduism is permeated by this phrase and theory.  When the three gunas are in equilibrium then matter—­Prakriti—­is quiescent, undifferentiated and unmanifested.  But as soon as the equilibrium is disturbed and one of the gunas becomes preponderant, then the process of differentiation and manifestation begins.  The disturbance of equilibrium is due to the action of the individual Purushas or souls on Prakriti, but this action is mechanical and due to proximity not to the volition of the souls and may be compared to the attraction of a magnet for iron.[744] Thus at the beginning of the evolutionary process we have quiescent matter in equilibrium:  over against this are souls innumerable, equally quiescent but exerting on matter a mechanical force.  This upsets the equilibrium and creates a movement which takes at first the form of development and later of decay and collapse.  Then matter returns to its quiescent state to be again excited by the Purushas and commence its world-making evolution anew.  The doctrine that evolution, dissolution and quiescence succeed one another periodically is an integral part of the Sankhya.[745]

The unmodified Prakriti stands first on the list of twenty-five principles.  When evolution begins it produces first Buddhi or intellect, secondly Ahamkara, which is perhaps best rendered by individuality, and next the five Tanmatras or subtle elements.  Buddhi, though meaning intellect, is used rather in the sense of ascertaining or perception.  It is the faculty by which we distinguish objects and perceive what they are.  It differs also from our conception of intellect in being, like Ahamkara and all the subsequent developments of Prakriti, material, and must not be confused with the immaterial Purusha or soul.  It is in fact the organ of thought, not in the sense of the brain or anything tangible, but a subtle substratum of all mental processes.  But in what sense is it possible to say that this Buddhi exists apart from individuals, who have not come into being at this stage of cosmic evolution?  This difficulty is not met by talking, as some commentators do, of cosmic as well as individual Buddhi, for even if all Prakriti is illuminated by Buddhi at this stage it is difficult to see what result can occur.  To make the process of development coherent we must think of it not as a series of chronologically successive stages but rather as a logically connected series and an analysis of completely evolved beings, just as we might say that