 Prima facie conceivability is one’s initial assessment of whether a described hypothesis is possible, without engaging in careful reasoning about the hypothesis. Better reasoning often gives one reason to doubt prima facie conceivability. Ideal conceivability, by contrast, involves a kind of reasoning that cannot be weakened by further reasoning. If some proposition is ideally conceivable, then that proposition is the case from the perspective of a hypothetical reasoner who is “free of all cognitive limitations” (Chalmers 2002: 148) and free from error due to lack of attention. For Chalmers this is sufficient for ideal reasoning to be safe from modal error. This is why, for Chalmers, an entailment link between conceivability and possibility can be forged only when ideal conceivability is in play.