Rene Descartes

Good sense is the most evenly shared thing in the world, for each of us thinks he is so well endowed with it that even those who are the hardest to please in all other respects are not in the habit of wanting more than they have.

...Because one cannot so well grasp a thing and make it one's own when it is learnt from another person, as when one discovers it oneself.

Let him deceive me who may, but he shall never be able to cause me to be nothing, so long as I think that I am something, or to cause it one day to be true that I have never been, it now being true that I am.

from Discourse on Methods and the Meditations.