wufwugy,
Think of it this way. Sasquatch contends that the universe might be infinite. Pookie contends that it is finite. This is all for the sake of example, btw, not accurate representations of their stances. Pookie says “The universe has boundary.” Sasquatch, our skeptic, says “How do you know?” Pookie says, “If the universe were infinite, the night sky would be blinding with the light from all of the stars.” Sasquatch replies “How do you know there isn’t dark matter, which absorbs the light, or that stars are evenly distributed through infinity? Further, how do you know that there is an edge to the universe? Have you ever been there, or seen it? If so, how can you trust your perceptions?”
The radical skeptic casts everything into doubt, including our senses. If we cannot trust our senses to reliably report our world, or if we may be dreaming and unaware of it, then our knowledge of the world is not entirely rational.
Now, as Leo Strauss put it, modern science has changed many beliefs from being simply untrue into untestable hypotheses. That is, science is pluaralistic, and attempts to be value-free. This is only an attempt, however, because science ceases to understand, for example, cultures as they understand themselves; rather it sees them through the lens of a separate culture. As such, they are NOT value-free. Anyway, this is drifting from the main point.
The important thing is that from a physical perspective, we can’t ever be “certain” of anything to the same extent as we are certain we exist. I would argue that ethics are a different realm, and that we CAN know some things about ethics that extend purely from the fact that we do exist, but that’s outside of the scope of this thread.
But all this being said, we are stuck with the world as it is presented to us, and so must make do with the scientific evidence as it becomes available. As pookie said earlier, scientists are forced to work with probabilities. Some hypotheses are unlikely to such an extent that their probability is near zero. The probability, for example, that I will transform into a penguin while typing this, is near zero. Logically, if all we have is physical evidence, and a theory is not supported by that evidence in any way, then that theory is either superfluous or wrong.