What can science prove?

[Frank]

I disagree. I'm simply saying a priori and posteriori knowledge doesn't require proof in the way you're demanding. The fact that tunafish exist is posteriori knowledge, and it is an allowable presumption we can base conclusions on.

And that's all it is. A presumption. It cannot be proven and hence anything based off it cannot be conclusive. As allowable or reasonable as the assumptions may intuitively be, the fact remains that we cannot prove tunafish to exist. Everything beyond "I think therefore I am" could simply be a delusion of the mind. It doesn't matter what is sensible or intuitive. The possibility is real and therefore science can never prove anything to be concrete and absolute beyond any reasonable (or unreasonable) doubt. As Adrian said, refer yourself to the China thread. It's all ridiculous and tedious but valid insofar as it shows that we cannot prove things scientifically.

That's a philosophical opinion, and not fact. Here's another hypothetical. Assume I'm in the supermarket right now, and I'm staring at a tunafish for sale in the sea food section, and I'm responding to this on my iphone (I'm not really, but assume I am). I would be able to say, at this instant, I'm absolutely sure tunafish exist on earth. Why, because I'm looking at a tunafish.

You might say I can't conclusively say that because I can't even be sure I exist. But I am sure I exist, because a priori knowledge (that rests on a priori justification) can be conclusive. You may take a different view, but nonetheless my view is valid and mainstream (although I acknowledge there's a variety of opinions on this). Moreover, there are statements that are absolutely true in all possible cases (e.g. necessary propositions).

I understand it's popular to say science can't conclusively prove anything; but pay special attention to "why" some people say that. That notion is predicated on the assumption that we can't be sure whether or not we really exist, much less rely on the uniformity of nature. However, it's only a technically accurate statement if the stated conclusion is overly broad; and you reject the validity of a priori knowledge (at least insofar as the conclusiveness of our existence). Once again there's differing views on this.

If a scientist was holding a tunafish and you asked him how sure he is that it's a tunafish he's holding, most would answer absolutely sure. If you asked your long haired philosophy professor (who has bongs hanging from his office ceiling), he will almost certainly give you a different answer; and you better pack a lunch - because it will be a very long and belated answer.

***oh yeah, I should have said "assumption" (not "presumption") in the previous post ... apologies.