Beginnings are tricky. I feel like I should make a good impression. I've had a haircut and a shave. I'm wearing a trendy shirt and some sort of designer smell. I'm going to start at the beginning...
As infants, each of us begins to form what a scientist would call “hypotheses”. Most, less pedantic, people would call them “theories”. That word has another meaning for the scientist, but I’ll come back to that. A hypothesis is an idea, a position we assume based on some limited information. Amongst the first hypotheses that infants will form goes something along the lines of “stuff falls down”. The gleeful child will test this new idea by experimentation. Hence the wondrous phase during which the child will pick up any and all objects within reach simply to give them forcefully to the floor. The reproducibility of this experiment convinces the child that the hypothesis is good and the idea finally graduates to a loftier status, probably giving birth to a new hypothesis in the process; “parent-things get angry when I make stuff fall down”.
Scientists adopt a similar method. Starting with some previously-solidified information, they adopt a hypothesis. Just as a child will do, until the complexity of life leads them to question the world less, they adopt an initial position, for the sake of argument if you like. The scientist has faith in one notion alone, that the universe may be tested by observation. So, the one essential requirement for a hypothesis to be valid is that it must be testable. The philosopher Karl Popper advocated going one step further, stating that the only valid hypotheses are those that could be falsified, that could possibly be proven wrong. A good scientist conducts his experiments, his means to observe the universe, in order to do just that. He seeks to falsify his hypothesis.
It is only when repeated attempts to prove himself wrong have failed that the scientist comes to believe his hypothesis at last. And it is only when those observations have been attacked and reproduced by many others, by colleagues and competitors, that the scientific community accepts the hypothesis. It is then that hypothesis becomes “theory”. There is in truth no such thing as scientific fact, at least not in the sense that most people imagine. Instead, theory is the highest status to which our knowledge may ascend. That difference between what scientists call theory and what the common man calls theory is the reason why a scientist will never say “it’s just a theory”.