Well, there's no way to PROVE the theory quantum mechanics, either. But if the theory of quantum mechanics WEREN'T true - or at least as close an approximation of reality as we could measure - the computers we're using to post and the server the posts sit on and the internet we pass the data over all wouldn't work.

There comes a time when the saw "correlation is not causation" falls apart.

It's just how statistically certain you want to be. The plot I posted earlier shows seven studies all showing helmet use signficantly decreases head injuries at a 95% confidence level. Now, those studies used different data and different methodologies, so the exact level of independence would be quite difficult to determine, but if those studies are all completely independent and could all be characterized as supporting the proposition "bike helmets reduce the chance of head injury" (and I argue that's EXACTLY what they do) you can come to the conclusion that there's a 99.9999999% chance that those studies "prove" the propostion that "bike helmets help prevent head injury".

Admittedly, that's not your specific statement regarding BRAIN injury, but it's pretty close, and it's hard to argue preventing head injuries in general won't prevent brain injuries specifically.

That's pretty doggone high - I'd say your odds of winning a few millions dollars in any lottery are much higher than the chances that bike helmets in fact provide no protection from head injury.

And guess what? Those seven studies that make up that chart aren't the only ones that conclude to a very high degree of certainty that bike helmets do indeed help prevent injury.

Of course, that doesn't mean mandatory helmet laws are acceptable. That's ANOTHER argument. But one that shouldn't be made based on junk science and an utter failure of logic. That just make MHL opponents look ridiculous.