Knowledge has traditionally been analyzed as justified true belief (the "JTB" analysis). That is, S knows that p if and only if:
- p is true
- S believes that p
- S's belief-that-p is justified
Despite being initially plausible, there are a class of counter-examples (known as "Gettier cases") which cast doubt on this analysis. The scenarios they describe satisfy all three conditions, yet leave us reluctant to ascribe knowledge to S. The identifying feature of a Gettier case is that the justification involved, though usually acceptable, in this case turns out to be corrupted somehow - so that it is merely a matter of luck that p turns out to be true.
The example our lecturer gave involved a man driving down a road. He sees a cowshed, and so (justifiably enough) forms the belief "there is a cowshed". And it happens to be true. However, unbeknownst to the driver, life-like facades have been put up all along that road. In fact, of the 100 "cowsheds" along that road, this one is the only real one. So surely he doesn't really know he's looking at a real cowshed after all?
Interestingly, our class' intuitions were split pretty evenly on that example. A great many people weren't convinced. But after the lecture, I was talking to a friend about Skepticism, and we unwittingly came up with the following Gettier case, which I think is much more convincing:
Suppose you see a tree nearby, and so (justifiably) form the belief "I am located within ten metres of a tree". But it turns out that you're really a brain in a vat (or dreaming, or in the Matrix, whatever...). However, by sheer chance, your vat happens to be positioned against the wall of the Brain-Vat Storage Warehouse, and there is a tree growing outside (within ten metres of your vat). So you have a justified, true belief. But I suspect most of us would be disinclined to say that you really know there is a tree nearby.