 Several replies are in order. First, Hume’s conceivability to possibility argument is unsound. For one thing, whose conceivability is being appealed to here? For another, someone who fails to understand a necessarily true proposition might conceive of it being false, but from this it does not follow that it possibly is false. A person might think (wrongly) that pi is a determinate number, but it does not follow that it is so. In the phenomenology of conceivability, what is really conceivable is difficult if not impossible to differentiate from what some might think is conceivable. And even if something is conceivable, say in a logical sense, it does not follow that it is metaphysically or factually possible. One might conceive that, since heads can be distinguished from tails on a coin, they can actually be separated, but metaphysically such is impossible. What is distinguishable is not necessarily separable. Hume, it seems, confuses epistemic with ontological conditions. Hence, the argument based on conceivability is suspect (Reichenbach 1972: 57–60).