Two scientists hold two different beliefs. Both are supported by the available data, but when new information comes in, one scientist is shown to really know what happened, while the other just guessed. But is that knowledge? A theoretical cow says no.
What's the difference between believing something and knowing it? Some people would say that the difference is proof. If someone is proved right, they didn't just believe something to be true, they knew it was true. Possibly.
Edmund Gettier, an American philosopher, was born in 1927, and so spent much of his life listening to different theories being confirmed or debunked. The proponents of some were exalted while the proponents of others were disparaged. While there's certainly a case to be made for some scientists correctly following data while their colleagues missed vital clues, does that necessarily make some famous visionaries "know" the answer while others just have false beliefs? When data can be interpreted multiple ways, every interpretation is equally valid. And some people have stumbled on the correct answer using false reasoning. We can be smug and say we "knew" we were right, but we weren't right at all.
To illustrate this Gettier came up with a thought experiment famously known as The Cow in the Field. A farmer sees that his cow is missing. As he's fretting, the mailman comes up and assures him that the cow is just in the next field over. The farmer wants to be sure, so he goes out and sees a familiar black and white splotch in the nearby field. He returns and says, yes, he now believes the cow is in the field. When the mailman passes back along that field, though, he realizes that the farmer couldn't have seen the cow. Yes, the cow was in the field, but it was hidden in a small grouping of trees. What the farmer actually saw was a piece of paper clinging to the outside of the trees that is splotched with black paint.
The cow was in the field, but was the farmer was wrong. His belief, as it turns out, was not justified, although it was understandable. He was wrong in his reasoning - and would have come to the wrong conclusion if the cow hadn't helped him out. Practically speaking, there is no difference. And it raises the bar pretty high. We come to conclusions based on inconclusive, though highly circumstantial, evidence every day. I think of the cow in the field with irritation when I hear people say that Edgar Allan Poe's prose poem, Eureka, predicted the Big Bang, even though he couldn't possibly have known about it. But I also think of it with compassion for those scientists whose beliefs, although smart and based on the best information available at the time, were not supported by history.