An enquiry into the relations of the mental phenomena to the physical has sometimes given the first start to philosophy. The relation of mind to matter is such an important problem of philosophy that the existing philosophical systems may roughly be classified according to the relative importance that has been attached to mind or to matter. There have been chemical, mechanical and biological conceptions which have ignored mind as a separate entity and have dogmatically affirmed it to be the product of matter only.[1] There have been theories of the other extreme, which have dispensed with matter altogether and have boldly affirmed that matter as such has no reality at all, and that thought is the only thing which can be called Real in the highest sense. All matter as such is non-Being or Māyā or Avidyā. There have been Nihilists like the Śūnyavādi Buddhists who have gone so far as to assert that neither matter nor mind exists. Some have asserted that matter is only thought externalized, some have regarded the principle of matter as the unknowable Thing-in-itself, some have regarded them as separate independent entities held within a higher reality called God, or as two of his attributes only, and some have regarded their difference as being only one of grades of intelligence, one merging slowly and imperceptibly into the other and held together in concord with each other by pre-established harmony. Underlying the metaphysics of the Yoga system of thought as taught by Patañjali and as elaborated by his commentators we find an acute analysis of matter and thought. Matter on the one hand, mind, the senses, and the ego on the other are regarded as nothing more than two different kinds of modifications of one primal cause, the Prakṛti. But the self-intelligent principle called Purusha (spirit) is distinguished from them. Matter consists only of three primal qualities or rather substantive entities, which he calls the Sattva or intelligence-stuff, Rajas or energy, and Tamas—the factor of obstruction or mass or inertia. It is extremely difficult truly to conceive of the nature of these three kinds of entities or Guṇas, as he calls them, when we consider that these three elements alone are regarded as composing all phenomena, mental and physical. In order to comprehend them rightly it will be necessary to grasp thoroughly the exact relation between the mental and the physical. What are the real points of agreement between the two? How can the same elements be said to behave in one case as the conceiver and in the other case as the conceived? Thus Vācaspati says:— “The reals (guṇas) have two forms, viz. the determiner or the perceiver, and the perceived or the determined. In the aspect of the determined or the perceived, the guṇas evolve themselves as the five infra-atomic potentials, the five gross elements and their compounds. In the aspect of perceiver or determiner, they form the modifications of the ego together with the senses.”[2]