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Perfect means "no change".
Knowledge means without any mistake. Anything without any mistake is perpetually right. And anything based on mistaken idea will change. You have to correct the mistake. One scientist is putting forward one theory—after a few years this theory is changed. Another person will say, "Now here is the perfect." After ten years, another will come," No, that is not perfect. This is perfect." This is going on. They take it as advancement in research, but actually the knowledge is imperfect. Otherwise, what is the necessity of research and advancement? They do not know what is the end of advancement. Therefore all their knowledge is imperfect. Imperfect knowledge is that as soon as he comes to the imperfect point, "yes, we are trying." Trying means still knowledge is imperfect. Now we have to accept knowledge from Vedas without change. We do not say, "Five thousand years ago Krishna said like this. Now we are advanced. We change this line." You cannot do that. If you change the word of the scripture, then where is the authority of the scripture? So as soon as you change, that means it is imperfect. Five thousand years ago Krishna said, "You give up all other religious principles. Simply surrender unto Me."[Bg. 18.66] We are preaching the same thing. One who is very experienced medical practitioner, he gives you a prescription; you visit him again and again, he gives you the same prescription unless you are cured. He doesn't change. He knows, "This is the disease, and ultimately this medicine will cure. So let him repeat that medicine." But a non-experienced physician, every time you go, he will change the prescription. We follow no change policy. Not that I think I have become advanced so, I change this to that.
Ref: Srila Prabhupada's Lecture, December 22, 1973, Los Angeles.