>But, in the end, we'll never be absolutely sure that one thing causes another. Maybe so, but in the same pedantic sense that we'll never be "absolutely sure"

>But, in the end, we'll never be absolutely sure that one thing causes another. Maybe so, but in the same pedantic sense that we'll never be "absolutely sure" that the universe didn't pop into existence five seconds ago with our memories already programmed in. This level of pedantry is great for impressing your fellow freshman philosophy students, but not actually useful for real life. For actual practical day-to-day purposes knowing whether something causes something else is really useful, so what can we do? Controlled experiments. If you want to know whether cyanide is poisonous, get a hundred undergrads, randomly select half of them, feed 'em cyanide and see which ones drop dead. If you observe a strong correlation between the variable you're interested in and the one you think you're controlling then you've established causation for all practical purposes. The pedant will point out there are two other possible explanations: 1. A massive coincidence -- those fifty undergrads just happened to drop dead for unrelated reasons. You can compute the probability of this as something like the average rate of spontaneous and inexplicable human death in undergrads to the power of fifty and multiplied by 1/(100 choose 50) [or something]. 2. A slightly less massive combined with a reverse-causal process. Fifty of your undergrads just happened to be about to die anyway, and some aura emanating from them affected your coin-flipping process. The probability of this is harder to quantify, though since the scenario still requires fifty cases of spontaneous undergraduate death in a short space of time we can put an upper bound on it.