Heat and light rays are supposed to consist of indefinitely small particles which dart forth or radiate in all directions rectilineally with inconceivable velocity.  Heat may penetrate through the interatomic space as in the case of the conduction of heat, as when water boils in a pot put on the fire; in cases of transparency light rays penetrate through the inter-atomic spaces with parispanda of the nature of deflection or refraction (tiryag-gamana).  In other cases heat rays may impinge on the atoms and rebound back—­which explains reflection.  Lastly heat may strike the atoms in a peculiar way, so as to break up their grouping, transform the physico-chemical characters of the atoms, and again recombine them, all by means of continual impact with inconceivable velocity, an operation which explains all cases of chemical combination [Footnote ref l].  Govardhana a later Nyaya writer says that paka means the combination of different kinds of heat.  The heat that

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[Footnote 1:  See Dr Seal’s Positive Sciences of the Hindus.]

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changes the colour of a fruit is different from that which generates or changes the taste.  Even when the colour and taste remain the same a particular kind of heat may change the smell.  When grass eaten by cows is broken up into atoms special kinds of heat-light rays change its old taste, colour, touch and smell into such forms as those that belong to milk [Footnote ref 1].

In the Nyaya-Vais`e@sika system all action of matter on matter is thus resolved into motion.  Conscious activity (prayatna) is distinguished from all forms of motion as against the Sa@mkhya doctrine which considered everything other than puru@sa (intelligence) to arise in the course of cosmic evolution and therefore to be subject to vibratory motion.

The Origin of Knowledge (Prama@na).

The manner in which knowledge originates is one of the most favourite topics of discussion in Indian philosophy.  We have already seen that Sa@mkhya-Yoga explained it by supposing that the buddhi (place of consciousness) assumed the form of the object of perception, and that the buddhi so transformed was then intelligized by the reflection of the pure intelligence or puru@sa.  The Jains regarded the origin of any knowledge as being due to a withdrawal of a veil of karma which was covering the all-intelligence of the self.

Nyaya-Vais`e@sika regarded all effects as being due to the assemblage of certain collocations which unconditionally, invariably, and immediately preceded these effects.  That collocation (samagri) which produced knowledge involved certain non-intelligent as well as intelligent elements and through their conjoint action uncontradicted and determinate knowledge was produced, and this collocation is thus called prama@na or the determining cause of the origin of knowledge [Footnote ref 2].  None of the separate elements composing