While preparing for comps, I came across a statement along the lines of "It is possible that Susan might not have been born. If you don't believe this, you are a philosopher with a theory."* But the issue seems so much more complicated than a dig at "theory-laden philosophers" would solve. I argue that our intuitions about which facts about the world are possible and which are necessary has relatively little to do with their actual status.
Take the following formula: 1,045,9879 * 230,840. What does it equal? Without stopping to work it out, does it possibly equal 2,414,557,468,360? It might seem like it could, but in fact, it doesn't and cannot possibly equal that amount. Our first intuitions about it don't mean much of anything. We must work through the chain of mathematical analysis, step by step, until we come to the answer.
We possibly get that problem wrong, but we don't ever think that 1+1=5. Why? Because there is only one step in the simple problem - it's almost impossible to misthink it. By contrast, there are many links in the chain of reasoning for the more complicated problem, and we don't immediately see all the links at once or how they are connected to each other.
"But," you might say, "we know that this is a mathematical problem, so we know that the answer must be necessary. But Susan's birth isn't math." Perhaps, but that doesn't mean that our intuitions about her possibly not-being are worth anything. It depends on whether physics is completely deterministic on a macro-level or probabilistic, on whether the initial state of the universal was necessary or whether it was random/intelligently designed/programmed by the great space potato/etc., on whether free human actions can alter the universe apart from deterministic laws, and so on. My intuition on Susan's birth isn't informed by the reality of any of those situations - that is, whatever makes me think of her possibly not being has nothing to do with my thoughts on these other matters. I think that her birth is merely possible, not because I actually understand anything about reality, but because I can't see the big picture to see how the complex web of causation operates. I see this little piece, and imagine it cut off from the whole, as if it actually could be, in the same way that I imagine that 1,045,9879 * 230,840 = 2,414,557,468,360 because I have not seen through all the steps.
In short, there seems to be no reason to leap from "I can conceive of x being possible/possibly not being," to "x must not be necessary." Our intuitions on states of affairs are completely bunk and should not be resorted to in our analysis of the world.
*: I found the quote. It is: "If Sally, an ordinary human being, says, 'I might not have existed,' almost everyone will take her to have stated an obvious truth. (Anyone who does not will almost certainly be a metaphysician with a theory.)" Van Inwagen, http://plato.stanford.edu/entries/metaphysics/
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