In plain words
not amenable to being changed. Why should this be so? Because all of them rest, and will always rest, upon the eternal laws of the human being and of Nature themselves. That is why they have taken shape as things that lie beyond change. Ideas concerning the Ātma never change: as they were thousands of years ago, so they remain today, and however many thousands of years may pass hereafter, they will remain exactly the same. Vedānta is that which holds one and the same principle, goal and rule of discipline across the past, the present and the future. Our religious customs and conventions, however, came into being resting solely upon our social conditions and the mutual relations we have with other communities, and so they inevitably change along with social conditions. What suits one age does not suit another. This is precisely why a food enjoined in one age comes to be prohibited in another: it had its meaning in that age. But because a land's climate and other such matters keep changing, and because so many new circumstances keep arising, changes were prescribed, in keeping with those shifts, in the Smriti's own rules on food and on other matters as well. Bearing this in mind, our society needs change to suit this modern age, and that change must indeed be carried out. But this does not mean that the Dharma of our religion will undergo so much as an atom's worth of change — they remain unshaken, and their bounds do not shift. Now one must try to come to understand the Purāṇas.
It is said that 'the Purāṇa has five defining characteristics' — that is, among them are matters of historical connection and matters concerning the order of creation. There are also ever so many philosophical tenets, clarified through technical illustration. These Purāṇas were composed in order to present the Veda's own subject-matter to ordinary people in a form they could understand. The language of the Veda is of an extremely ancient form.