I haven't read the original paper, so maybe the example is better, but it seems the cow example fails the justified condition. The knowledge is justified if it derives from the evidence, but once we know the evidence is faulty it can no longer be used for justification by definition. It seems by extension that any justified true belief can become unjustified by the addition of new information that invalidates the justification on which the alleged knowledge is based upon.
What you're saying is more or less exactly what the paper was getting at.
It's hard to say based on a short Internet comment, but it sounds like the spot where your disagreement comes from is that you're understanding the word "justified" in a slightly different way from how epistemologists were using it. For example, one of the responses to Gettier's paper was to suggest that maybe the definition of "justified" should be altered to include a provision that invalidating the justification would imply that the belief is false.
So, for example, under that modified definition, the visual evidence couldn't serve as a justification of the belief that there is a cow in the field, because it allows the possibility that it isn't a cow but there still is one in the field. On the other hand, it would work for justifying a belief like, "I can see a cow from here." (Yeah, there's another cow in the field, but it's not the one you think you see.) But, still, that wasn't quite the definition that the mid-century epistemologists who made up Gettier's audience were using.
Thanks, I think you're right about how I was understanding the word "justified". I like bringing up philosophical disagreements on HN since it often gets responses like yours :)
Yeah, sorry, though, I realized after I posted that I failed to properly acknowledge that you hit the nail on the head -- I picked that response to Gettier specifically because it matched your criticism.
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