Listen to part of a lecture in a philosophy class.

Okay. So, uh, to continue our discussion...­ When philosophers talk about the basis of knowledge, they don't mean the source of information about any particular subject. They mean how we know what we know. 

Let's start with one philosophical view - foundationalism. 

Foundationalism is the view that our knowledge claims, what we think we know, that is, they need to have a base. And think of knowledge as a house, you need a solid foundation on which to build your house.And if you have a strong foundation, your house is more likely to be solid. Well, foundationalists think the same thing is true of knowledge.If you have a solid base for your knowledge claims, then your knowledge structure is more likely to be strong valid, true. 

First, you need some good foundational knowledge claims, and then the rest of the knowledge claims can be based on these. Now, as to what kinds of knowledge claims are foundational, well, that's where this gets particularly interesting, in fact it sort of depends on which philosopher you ask. Take John Locke for instance. 

Locke's viewpoint essentially was that when humans are born, their minds are like blank slates, that is, we don't have any kind of knowledge when we are born. We get our knowledge from our senses, you know, taste, touch, smell, sight, hearing. So, when we look at the world, first as babies and then as we grow, that's where our knowledge comes from.  Our senses, our experiences serve as the foundation for our knowledge.

Now, for a very different view, let's turn to another philosopher - René Descartes. 

Descartes thought that you have to go much deeper to find the foundations.He believed that our senses are not to be trusted.So he wanted to find a more solid foundation for knowledge. He began with what has come to be called methodological doubt.And when we say methodological doubt, well...­Descartes believed that everything should be questioned, that is, approach it with doubt, and that 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.