For any claim X, exactly one of these is true:
1) X is possible - compatible with the evidence so far.
2) X is impossible - there's a contradiction between X and the evidence so far.
That means asserting the possibility of something is harder than you think. For example, if Bob says: "Epiphenomenalism is possible, therefore <far reaching conclusions>"
Wait, Bob, did you just say it's impossible to find a contradiction between epiphenomenalism and anything else you know? That's an awfully strong claim! You got any evidence?
Bob pulls back: "I meant only that X sounds plausible to me." But that's a fact about Bob's limits of reasoning, it doesn't support the far-reaching conclusions anymore. For that you need to justify (1) over (2), not just assert it.