Quantum mechanics tells us that the universe operates in very, very unusual ways. It is natural to want to dismiss some of the interpretations of QM (such as that natural laws are inherently probabilistic) as problems with our understanding, not as genuine features of reality. This is essentially a knee-jerk reaction to the oddness of QM and how it does not "mesh" with our everyday understanding of the world. However, science also informs us that we are the products of many years of evolution--and this evolution process equipped us to deal with one thing, and one thing only: survival. Our senses and reasoning faculties were not "designed" to help us humans apprehend truths about the world; rather, they were selected to enable humans qua systems to manipulate information in such a way that the systems preserve and replicate themselves. Luckily, knowing the truth--or approximating it--proved beneficial to the survival of these systems (these information processing patterns).
In simpler terms, more proto-humans who were able to correctly judge that there really is a savage tiger hiding in the grass over there successfully passed on their genes than those who hallucinated constantly. Or than those who had no knowledge whatsoever, who were not able to act on their knowledge, who had faulty reasoning processes, whatever.
Thus evolution tells us.
Unfortunately, to say "there really is a tiger over there" and leave it at that drastically oversimplifies the state of human existence. Because really, we don't know that there is or is not a tiger over there. All we know is that some collection of sights, sounds, scents, or inferences has caused us to believe that something over there can cause damage to us if we do not act accordingly. (Assume it's a hungry tiger and that we are defenseless in a savanna.) We may think to ourselves "There is a tiger," and we may believe "There is a tiger," and there is probably even a sense in which it is true that there is a tiger. But what we call a tiger is a convenient abstraction--a shorthand tag which bundles together a collection of concepts, memories, and/or feelings. So too, presumably, with our notion of existence--we have a certain understanding of what it means for something "to be" and "to be there." and we predicate this notion upon the abstraction "tiger."
[Edit as of 04-05-2009: I should mention that yes, Kant (and other philosophers) claim that existence is not a predicate. I do recognize that it's a contested notion and I disagree with the mainstream opinion; but I shan't defend it now.]
"Well, what of it?" I hear you say. "You're not telling us anything that the pioneering philosophers of the 17th-18th century didn't when they first speculated about our psychological workings; and you're barely even consistent with today's psychological theories."
Fair enough. But, if we can suppose that the tenets of natural selection are true, then we should firmly keep in mind that our knowledge...
This post is dissolving into incoherency. And as usual I don't have the patience to fix it. I shouldn't even post it. But whatever.