Philosopher Gilbert Harman examines the Gettier cases in his book Thought. From the Gettier cases, Harman thinks we can glean the following principle: "Reasoning that essentially involves false conclusions, intermediate or final, cannot give one knowledge." That is, in the course of one's reasoning, a person can't come to a conclusion that is false and that belief really be knowledge. Harman doesn't want to argue with this principle. He thinks this principle is true. But he wants to find some way to accept the principle and address the Gettier cases.
While looking at the Gettier cases, though, he indirectly addresses and challenges Klein's argument. Harman gives three examples, but I'll just address one, and I'll try to simplify it. The first is about Tom, a guy you know. You know Tom, and you saw Tom stealing from the library. You tell his mother but you hear from Tom's mother that Tom has an identical twin Buck and he loves to steal books. You don't know much about Tom's family so you assume this is true. Another friend tells you later that Tom's mother is insane. And now you don't feel so sure about whether Tom stole the book or not. But had you never heard the wrong information you would never have had the doubt. And so do you not really know what you thought you knew?
Harman wants to avoid paradox about knowledge, and he defines the paradox like this:
"If I know that h is true, I know that any evidence against h is evidence against something that is true; so I know that such evidence is misleading. But I should disregard evidence that I know is misleading. So, once I know that h is true, I am in a position to disregard any future evidence that seems to tell against h." This is paradoxical, because I am never in a position simply to disregard any future evidence even though I do know a great many different things.Given that you knew it was Tom before you heard the info before, and given that you relied on the testimony of Tom's mother later, though it was wrong, you can nevertheless be confident that you knew it all along, although you had a doubt in light of the testimony.
So Harman squares off with Klein here. He challenges Klein's fourth condition for knowledge and says that if we accepted it we would have to reject that people know things in situations of new true propositions like in the counterexample, like for example the true sentence that Tom's mother testified contrary to what you knew or the true sentence that in similar situations a person would usually trust someone's mother. Harman thinks in such situations we actually would know Tom stole the book in spite of the new information that created doubt, whereas Klein, if he really accepted his four conditions for knowledge, would have to reject it.
Ultimately, Harman reconfigures the definition of knowledge. It looks something like this.
(i) p is true;
(ii) S believes p at t1;
(iii) p is evident to S at t1;
(iv) One's conclusion that p is not based solely on reasoning that essentially involves false intermediate conclusions.
So, with this principle, Harman can have his counterexamples and his definition of knowledge too.
I hope I explained this well. What do you think? And do you think this a better definition of knowledge?