A question naturally arises, that if the knowledge forms are made up of some sort of stuff as the objective forms of matter are, why then should the puru@sa illuminate it and not external material objects. The answer that Sa@mkhya gives is that the knowledge-complexes are certainly different from external objects in this, that they are far subtler and have a preponderance of a special quality of plasticity and translucence (_sattva_), which resembles the light of puru@sa, and is thus fit for reflecting and absorbing the light of the puru@sa. The two principal characteristics of external gross matter are mass and energy. But it has also the other characteristic of allowing itself to be photographed by our mind; this thought-photograph of matter has again the special privilege of being so translucent as to be able to catch the reflection of the _cit_--the super-translucent transcendent principle of intelligence. The fundamental characteristic of external gross matter is its mass; energy is common to both gross matter and the subtle thought-stuff. But mass is at its lowest minimum in thought-stuff, whereas the capacity of translucence, or what may be otherwise designated as the intelligence-stuff, is at its highest in thought-stuff. But if the gross matter had none of the characteristics of translucence that thought possesses, it could not have made itself an object of thought; for thought transforms itself into the shape, colour, and other characteristics of the thing which has been made its object. Thought could not have copied the matter, if the matter did not possess some of the essential substances of which the copy was made up. But this plastic entity (_sattva_) which is so predominant in thought is at its lowest limit of subordination in matter. Similarly mass is not noticed in thought, but some such notions as are associated with mass may be discernible in thought; thus the images of thought are limited, separate, have movement, and have more or less clear cut forms. The images do not extend in space, but they can represent space. The translucent and plastic element of thought (_sattva_) in association with movement (_rajas_) would have resulted in a simultaneous revelation of all objects; it is on account of mass or tendency of obstruction (_tamas_) that knowledge proceeds from image to image and discloses things in a successive manner. The buddhi (thought-stuff) holds within it all knowledge immersed as it were in utter darkness, and actual knowledge comes before our view as though by the removal of the darkness or veil, by the reflection of the light of the puru@sa. This characteristic of knowledge, that all its stores are hidden as if lost at any moment, and only one picture or idea comes at a time to the arena of revelation, demonstrates that in knowledge there is a factor of obstruction which manifests itself in its full actuality in gross matter as mass. Thus both thought and gross matter are made up of three elements, a plasticity of intelligence-stuff (_sattva_), energy-stuff (_rajas_), and mass-stuff (_tamas_), or the factor of obstruction. Of these the last two are predominant in gross matter and the first two in thought.