It's been realized recently that the vast, vast majority of the bacteria that live in the body can't be cultured using current techniques. Decades (a blip on human history, of course) have been dedicated to the study of the stuff we can grow in a petrie dish, and the odd thing we've been aware of that can't be grown -- Mycobacterium leprae, for example -- has been thought of as just that -- an odd thing. Which is perfectly understandable, of course: if you can only read braille, there's no point in fretting over the pictures in a coffee table book.
But now that we've got a means of reading coffee table books, we see that the stuff we've been looking at so far are, to some extent, the odd bits -- wee bugs so hearty you can rip them out of their milieu, dump them on the microbial equivalent of junk food, and they thrive.
Quote:Humans have often decided that there is a limit to the depth of knowledge about some natural phenomenon and that it has been reached.
Of course, to act in the real world, you've got to behave as though you've got a pretty solid picture of the world, even if you don't. And most of what Newton figured out will still suffice even for most engineers, even if it's all turned out to be "wrong."