Some scientists like to think that they pressume nothing, and have the cold pursuit of facts as their only rule. In fact this is largely nonsense. If we really stop to think about it we realise that all knowledge is based on certain assumptions and approximations. In lots of cases (most, even) these assumptions are reliable and good, but what happens when something occurs which contradicts all our percieved limitations?
A few years back a man called Thomas Kuhn demonstrated what can happen by cheekily constructing a deck of playing cards in which the suits of hearts and diamonds were coloured black and the clubs and spades were red - this of course going against the whole mind set you automatically take on as soon as you glimpse a pack of small cards with that characteristic pattern on their backs. Kuhn conducted an experiment where he quickly flashed random cards at his subjects, who he asked to try and identify them. He found most people identified the black cards as either spades or clubs automatically, even though they weren't, but displayed some degree of brow-furrowing. The second time round people were utterly confused and some even became angry. He even cited one of his subjects as crying and screaming hysterically, such was her distress at realising that somehow everything she thought she knew about cards was suddenly and inexplicably wrong with no apparent explanantion.
Kuhn conducted this experiment in order to try and illustrate his theory of 'paradigm shifts' and wrote it up in his 1962 book The Structure of Scientific Revoloutions. A historian by trade, Kuhn believed that scientists (like all of us) make a set of assumptions about their field of research. This could be based on the previous work done, where and by whom you have been taught or just on random prejudices. Generally this set of rules is shared by the majority of informed people in a field and forms what Kuhn dubbed a paradigm. A paradigm then, is like a window, the assumptions we make about things like the sills which frame the view. A paradigm shift is what happens when suddenly someone finds out something which they can't explain using the current scientifc theories - the window frame shifts to make room for new ideas.
Paradigm shifts have occured on a global scale lots of times in history as Kuhn expertly details in his book. For example it must have been one heck of a shock when one day a bloke called Nicolaus Copernicus came along and told everyone that in fact it was not the earth which was at the centre of the universe, but the sun (of course, we now know that's not exactly the full picture either). Equally, when scientists such as Bohr, Einstein and Planck started observing that the laws of nature which work for very large objects (like footballs and planets) didn't work for very small objects (like electrons and atoms) they found they needed new laws to explain what they were seeing. We now call that quantum mechanics.
Some argue that we are currently moving from a modern to a post-modern era and this is a form of cultural paradigm shift. Modernism was characterised by the formation of large, characterless conglomerations and as a result the making of large sums of money. Post-modernism is rather the opposite: a distrust of organised bodies of all kinds (especially the government and especially the financial systems) and a search for a more personal and resonant meaning and truth.
There are paradigm shifts going on all over the place. Now you know what they are you can try and spot one and tell someone else. You'll make yourself look very intellectual. Either that or just appear wierd and alienate yourself a little.