We have just seen that Sāṃkhya does not admit the existence of God, and considers that salvation can be obtained only by a steady perseverance in philosophical thinking, and does not put emphasis on the practical exercises which are regarded as essential by the Yoga. One other point of difference ought to be noted with regard to the conception of avidyā. According to Yoga, avidyā, as we have already explained it, means positive untrue beliefs such as believing the impure, uneternal, sorrow, and non-self to be the pure eternal, pleasure and the self respectively. With Sāṃkhya, however, avidyā is only the non-distinction of the difference between prakṛti and purusha. Both Sāṃkhya and Yoga admit that our bondage to prakṛti is due to an illusion or ignorance (avidyā), but Sāṃkhya holds the akhyāti theory which regards non-distinction of the difference as the cause of illusion whereas the Yoga holds the anyathākhyāti theory which regards positive misapprehension of the one as the other to be the cause of illusion. We have already referred to the difference in the course of the evolution of the categories as held by Sāṃkhya and Yoga. This also accounts for the difference between the technical terms of prakṛti, vikṛti and prakṛti-vikṛti of Sāṃkhya and the viśesha and aviśesha of the Yoga. The doctrine of dharma, lakshaṇa and avasthāpariṇāma, though not in any way antagonistic to Sāṃkhya, is not so definitely described as in the Yoga. Some scholars think that Sāṃkhya did not believe in atoms as Yoga did. But though the word paramāṇu has not been mentioned in the _Kārikā_, it does not seem that Sāṃkhya did not believe in atoms; and we have already noticed that Bhikshu considers the word sūkshma in _Kārikā_ 39 as referring to the atoms. There are also slight differences with regard to the process involved in perception and this has been dealt with in my _Yoga philosophy in relation to other Indian systems of thought_.[43] On almost all other fundamental points Sāṃkhya and Yoga are in complete agreement. CHAPTER XV MATTER AND MIND In conclusion it may be worth while saying a few words as to theories of the physical world supplementary to the views that have already been stated above.