Most of our knowledge stands in thin air...
Do you think that you know something? How do you know that it is true? How certain are you? Let's imagine a conversation between you and Descartes*.
You: "Hi philosopher! Do you know anything about what happened to Donald's broken leg?"
Descartes: "How do you know that Donald's leg is broken?"
You: "They showed him on TV. He looked pathetic walking with his broken leg."
Descartes: "How do you know that what your TV recieved was correct? And that what you saw was what the TV showed?"
You: "Hey! My TV set has always been receiving the correct transmission all the time. And your second question is a crazy one. The answer is obvious!"
Descartes: "So you believe your TV set because it behaved well in the past, huh? Don't be so sure about it - you are using, after all, an inductive method only. And I don't think anything at all is obvious. Why do you take something as true without proving it?"
You(slightly dazed): "But don't we assume lots of things without proof? Why, I can't even be sure that you are here if I don't assume that my eyes to tell me the truth. But how can I prove that they are? No, I'll have to assume that my eyes tell me the truth."
Descartes: "You might equally well assume that your eyes tell you the exact opposite of what is happening, can't you? You don't have a reason for believing only one among these."
You(with reeling mind): "Well, yes. What's the point of all this? Why on earth should I believe that my eyes are telling me the opposite of what's true?"
Descartes: "You actually need not. But that, I believe, made something clear to you: What you see as true depends upon what you assume to be true. There is no point in arguing if your basic assumptions are different."
You: "Agreed. But what if I can argue and make the other person take my assumptions as true?"
Descartes: "That you cannot do by logical argument, because your assumptions are not based on any reason. You will have to resort to emotional argument: by 'threat' or 'promise'! "
You: "So you say that reason has to be based upon unreason? Logic is always based upon illogical assumptions? What the heck do I believe then, if I cannot believe in any logical proof?"
Descates: "Think for yourself! But one thing you can believe is that it is good to remove the pain of others. Because pain is, by definition, something to be removed. So do that, that is the only thing worth doing."
*Note: I have used Descartes' name here only to represent the model philosopher. The above argument may or may not accurately represent his points of view.