The original name of Vyas Munni was Krishan Dwipayan son of Rishi Prashar and mother Satyawati. He did tapsya and learnt four Vedas traditionally from Rishi and enlighted. At that time no other modren religions raised and there was only four Vedas the knowledge direct from God originated in the heart of Agni, Vaayu, Aditya and Angira originated at the time of creation. Vedas knowledge from mouth to mouth and heart to heart. That is why Patanjali Rishi in his Yoga shashtra says in Sutra 1/26 that Almighty God is the first Guru of our ancient Rishis named above. Vedas are not books this knowledge was mouth to mouth by heart. First time in Dwapur approximately 5300 years back Vyas Muni wrote the Vedas on Bhoj patra and when press came then Vedas were published. It is a fundamental law that untill knowledge is given it cannot be attained by others. When first time universe is created then there become no Guru who could give knowledge to the whole illterate personal of the universe. So God is our first Guru who gives Vedas knowledge to four Rishis and uptill now the Rishis, Munis who knew and knows (at present) Vedas becomes our Gurus. This is fundamental and can not be changed at our own accord. Kapil Muni in Samkhya shastra sutra 2/25 clearly says that the truth based on eternal proof from Vedas can not be denied merely based on our own baseless thinking.
Still if we nurse new born baby in a densy cave of a jungle and do not give him any knowledge then even in the age of 25 or thirty he will not be able to speak and to tell any name of materialistic articles mother or father. Because he was not taught. At the time of creation the whole man or woman in the unsexual world become totally illiterate because the learned Rishi Muni of previous universe which is destroyed are not available in the new creation. So only Almighty God gives knowledge of Vedas to the four Rishis named above. This is fundamental and but natural under law of God at the time of eachcreation. Thus the Rishis studied the Vedas traditionally and got knowledge about Prakriti from whom world is created, jivatma(soul) and God. And endless knowledge of every matter in the universe mentioned in Vedas so our previous Rishis were real scientist too. After getting knowledge grom Vedas the six shashtras, upnishads, Bhagwat Geeta and other holy books were written.
1. Nyay shashtra is written by Mahrishi Gautam. This shashtra has five chapters (538 sutras) in which he has explained 16 matter for obtaining final liberation by which we reach to our motto that is called Nyay. Nyay shashtra mostly explains general knowledge and solutions of problems.Details about soul and salvation and the knowledge of the Almighty God. Butmainly this shashtra says about 16 matters as a proof cited above. Now a days mostly the saints do not give proof of Vedas, mantras whatever they say based on their study and listening which is sin. Manu Smriti shalok 255 chapter 4 says —-YEHA ANYATHA SANTMATMANAMANYATHA——that he who says baselessly (without proof of Vedas) he is thief of his own soul. So based on the knowledge of the four Vedas Rishis wrote shashtras etc., otherwise they could not.
2. Vaishashik shashtra is written by Maharishi Kannad. Rishi means he who has got full knowledge of four Vedas and ashtang yoga but it is astonished that most of the modern saints do not know abc about yoga and Vedas but they write themselves Braham Rishi. Vaishashik Shashtra has 10 chapters (370 sutras). In this shashtra details of six matters, nine dravya, 14 qualities five type of karmas etc., are given and it says that dharms that by which param sukh (pleasure) of this birth and after leaving this body (lok and parlok) is attained.
3. Samkhya shashtra is written by Kapil Muni (6 chapters and 451 sutras) which says about Prakriti, Raj ,tam, sat guna describing 25 main matters. And the main purpose of our birth is to kill sorrows, problems tensions,etc., ( three types of taap i.e., sorrows) to attain salvation.
4. Yoga shashtra is written by Patanjali Rishi (4 paad and 194 sutras) which explains about ashtang yoga i.e., yam, niyam, asan, pranayam, pratyahar, dharna, dhayan and samadhi, the real way to attain the salvation.
5. Mimansa shashtra is written by Maharishi Gaimini (12 chapters 60 paad, 2731 sutras). This shashtra says that religion is only known by studying the Vedas because Vedas are the self proof being eternal knowledge of Almighty God.
6. Vedanta shashtra is written by Maharishi Vyas (4 chapters, 555 sutras) this shastra says about God. Vedanta means the last chapter of Vedas. It says that we all human beings must follow the right path of Vedas to do worship of Almighty God to get final liberation.
So main object of all the six shashtras is to realise God while discharging moral duties and the knowledge in these shashtras is 100% from four Vedas which every Rishi has told in his respective shashtras. The subjects are different but the motto andviews of all the shashras are one i.e., to realise God which is main motto of four Vedas. Vedas have so many subjects but Rishis has taken one subject from Vedas in their respective shashtra. So shashtras are not contradictory as usually most of the modren saints say. Full knowledge of the Vedas can not be written in one life being so vast and deep so every Rishi haschosen one small subject from Veda’s vast knowledge. There is no any modren shashtra written by anybody else who is a philosopher of Vedas like Vyas or Kapil Muni etc.
Beneficial are only four Vedas, thereafter shashtras etc.