Je pense donc je suis - René Descartes

I will follow this strategy until I discover something that is certain or, at least, until I discover that it is certain only that nothing is certain.

Suppose [a person] had a basket full of apples and, being worried that some of the apples were rotten, wanted to take out the rotten ones to prevent the rot spreading. How would he proceed? Would he not begin by tipping the whole lot out of the basket? And would not the next step be to cast his eye over each apple in turn, and pick up and put back in the basket only those he saw to be sound, leaving the others? In just the same way, those who have never philosophized correctly have various opinions in their minds which they have begun to store up since childhood, and which they therefore have reason to believe may in many cases be false. They then attempt to separate the false beliefs from the others, so as to prevent their contaminating the rest and making the whole lot uncertain. Now the best way they can accomplish this is to reject all their beliefs together in one go, as if they were all uncertain and false. They can then go over each belief in turn and re-adopt only those which they recognize to be true and indubitable.

Η κοινή λογική, δηλαδή, ο ανθρώπινος λόγος, αφορά την ικανότητα διαμόρφωσης ορθής κρίσης και διάκρισης του σωστού από το λάθος. Τη συναντάμε εξίσου σε όλους τους ανθρώπους, και διαμορφώνεται μόνο με βάση το πως χρησιμοποιείται