Brāhmaṇa means knowledge and śūdra means no knowledge, that is the difference. One who knows... Just like in this age there are so many scientists, so many philosophers, but they have no perfect knowledge. Therefore they are śūdras. One scientist putting forward one theory—after a few years this theory is changed. That means knowledge is not perfect. They take it as advancement in research, but actually the knowledge is imperfect. Otherwise, where is the necessity of research and advancement? Advancement means you are in the lower grade. So all their advancement, the same lower grade. Because it is going on, they do not know what is the end of advancement. Therefore all their knowledge is imperfect; they are all śūdras.
We cannot accept knowledge from śūdras. Knowledge must be taken from a brāhmaṇa.