The Gettier problem has shown that knowledge is not simply justified true belief. This leads to the following principle P: reasoning that essentially involves false conclusions, intermediate or final, cannot give knowledge. So, we cannot argue from the false intermediate conclusion.

Undermining evidence

Gilbert Harman builds into this principle a new condition Q: one may infer a conclusion only if one infers that there is no undermining evidence that one does not possess. It is not necessarily that undermining evidence has to be true. This account of knowledge relies on two examples.

Stolen book case

I see Tom stole the book from the library. I am sure that it was Tom. I give the testimony at the Court. Later, Tom’s mother testifies that Tom has an identical twin, Buck. She says that Tom was thousands of miles away at the time of the theft. She hopes that Buck did not do it, but she admits that he has a bad character. In these circumstances, I do not know that Tom stole the book. My knowledge is undermined by evidence I do not possess ( even if his mother was lying when she said that Tom was thousands of miles away and the twin was a figment of her imagination).

Letter from San Francisco case

I saw my friend at the airport and he said that he was going to Italy for the summer. That was in June. It is now July. For some reasons, my friend wants me to believe that he is not in Italy but in California. He writes several letters saying that he has gone to San Francisco. He sends letters to someone he knows there and has that person mail them to me with a San Francisco postmark. I am now standing before the pile of mail that arrived while I was away. If I was asked at this moment “Do you know where is your friend?”, I would answer “Yes”, and I would be right that he is in Italy. Nevertheless, my justification for believing where my friend is makes no reference to letters from San Francisco. Hence, I don’t know that my friend is in Italy, because my knowledge is undermined by evidence I do not possess.

Conclusion

So, the principle of Harman’s knowledge is as follows.

h = [ r & no undermining evidence that one does not possess ] , where

  • h is a final conclusion
  • r is an intermediate conclusion (Tom stole the book, Friend is in Italy)

Even though r is true (intermediate conclusion), the final conclusion h can be false if there is undermining evidence that you don’t possess. You have an obligation to seek out an evidence that may undermine your conclusion. You need to believe that you have taken all right precautions that there are no undermining evidences at all.

Bibliography:

  • Gilbert Harman, Reasoning and Evidence One Does Not Possess, Midwest Studies in Philosophy, 1980