The first discourse of Descartes's Discourse on Method begins
Good sense is mankind's most equitably divided endowment, for everyone thinks that he is so abundantly provided with it that even those with the most insatiable appetites and most difficult to please in other ways do not usually want more than they have of this. As it is not likely that everyone is mistaken, this evidence shows that the ability to judge correctly, and to distinguish the true from the false--which is really meant by good sense or reason--is the same by innate nature in all men, and that differences of opinion are not due to differences in intelligence, but merely to the fact that we use different approaches and consider different things.
I remember my surprise on first reading this, something over forty-five years ago, and my admiration for the argument, combined with a feeling that something was wrong with it. This was long before anyone talked about the Dunning-Kruger effect. And really the Dunning-Kruger effect does not address Descartes's point as it affects the Discourse, for thoroughly intelligent persons of, before, and since his time have come to conclusions far different from his.
My old copy of The Discourse on Method and Meditations was lost in some move long ago, and I have just bought a used copy. The last time before yesterday that I saw the first sentence in print was in some Oracle documentation, in the days when one got a shelf's worth of printed manuals along with the software. The manual must have been Oracle Database Concepts, I think. Now all Oracle documentation is on-line, which for most purposes is much handier. But the pithy quotations at the beginning of chapters are no longer there.