You may believe something, fine, and have whatever justifications you wish. But how do you know the thing is true?
The point he was making was that far beyond the issue of problems in having the right sort of justifications is the problem of having truth as well. Whenever the Justified-true-belief scheme is used for knowledge the truth of the thing in question is whitewashed over: all the focus is put on the justification and the truth is assumed to exist separately.
For example if I make a claim P, then I clearly believe P, I will need to give justifications x, y, z, etc., and P needs to be true for me to count P to be part of my knowledge. The first two conditions are easy enough for me to demonstrate according to some standards, even if skepticism is still an issue. However, I, nor anyone else, has any ability to demonstrate the truth of P in ways over and above whatever I have given as my justification. Therefore Justified-true-belief reduces to Justified-belief, which no one accepts as knowledge.
Between this argument and Gettier, I see the Justified-true-belief scheme of knowledge as beyond saving. To recover some sense of knowledge, we can focus on this idea:
If you know something, then it is not possible to be mistaken.
There are two ways of dealing with this conditional. First, you can make your definition of what it is to know something always correspond with whatever you cannot be mistaken about. Besides being ad hoc, this sliding scale for knowledge does not correspond very well with what we generally take to be knowledge.
Secondly, we can make what it is not possible to be mistaken about correspond to our knowledge. Although you have already called foul, hear me out. If you were to find out certain things were wrong you might start to doubt your own sanity. For example if you were to find out all the basic things you ‘know’ were wrong – there is no such place as the United States, water is not comprised of oxygen and hydrogen, subjects and verbs are one and the same, you are currently not reading, etc., – you would have reason to worry (at least I would).
Therefore I suggest that knowledge is comprised of things that if they were to be false, then we would not be able to claim we were sane. This definition makes a distinction between things we can be mistaken about and things we cannot be mistaken about. To be mistaken about this second type of thing would entail an unacceptable consequence: if you are insane then you cannot claim to have knowledge.
Is this ad hoc, as above? No, because the definition of what would classify you as insane does not refer to knowledge specifically. For example take the statement, “If x, y and z are false then I am crazy.” No mention of knowledge whatsoever. Therefore this definition is not ad hoc.
Does this definition of knowledge correspond to our intuitions? Very much so: it is based specifically upon the everyday experiences we have and our most established theories of the world.
What about skepticism: can’t we always be mistaken? The skeptic here is asking us to imagine the unimaginable. If we do as the skeptic asks, then we would be required to imagine ourselves to be insane and tell the skeptic what we think as insane people. I can’t do this- I don’t even have a guess as to how to go about trying to do this.
In the end you are wagering your sanity in order to have a claim to knowledge. However, there is no danger in this bet because you hold all the cards: you know what you can imagine to be different. Therefore you gain a theory of knowledge and lose nothing.