And when we say methodological doubt, well … Descartes believed that everything should be questioned, that is, approach it with doubt
And if you could find one thing that cannot be false, that one thing would serve as a foundation for all other knowledge claims.
So unlike John Locke, Descartes doubts that knowledge comes to him from his senses.
He points out that at some time or another, everyone has been deceived by their senses.
We have all had experiences where our senses have been wrong—illusions, perhaps, mirages.
When driving in a car on a hot summer day, you may see what looks like shimmering water on the road, which, as science tells us, is really just a mirage, an illusion caused by the heating of the air.
Our senses are wrong, they've deceived us.
And Descartes thinks that since our senses can deceive us, we ought not take for granted that what they tell us is really true.
That's the first step in his methodological doubt.
From there he wonders, well, ok, I can doubt my senses, but can I doubt that I am sitting in this room?
Can it seem that we are not really here, that we are somewhere else?
He conceives that most of us would know that we are sitting in the room.
But then he says, well, couldn't I just be dreaming?
He's had dreams that were so real that he thought he was awake when in fact he was actually asleep.
And this is another good point—it's really hard to be sure that you are not actually dreaming.
Yet another proof for Descartes that we can't always trust what our senses are apparently telling us: we could be dreaming.
And there's really no good way to prove that we are not.
So the common sense picture of reality, that the world is really the way it looks to us, Descartes shows that we cannot just assume this to be true beyond all doubt.
And he does this by talking about illusions and also by arguing that we could be dreaming.
"But consider this," he says, "while one is thinking or doubting, or doing any of those sorts of mental activities, one has to exist, right?"
To even think that I doubt that I exist, you have to exist!
And so what Descartes has done is find at least one thing that he can be certain of.
He says, "I exist." And that's a start.
And other knowledge he tells us can be based on that foundation.