Are there a posteriori conceptual necessities?

I critically assess Stephen Yablo's claim that "cassinis are ovals" is an a posteriori conceptual necessity. One does not know it simply by mastering the relevant concepts but by substantial empirical scrutiny. Yablo represents narrow content by "would have turned out"-conditionals. An epistemic reading of such conditionals does not bear Yablo's claim. Two metaphysically laden readings are considered. In one reading, Yablo's conditionals test under what circumstances concepts remain the same while their extensions diverge. As an alternative, I develop a more literal metaphysical interpretation: Yablo's conditionals draw on scenarios which are qualitatively identical to some original situation. None of these interpretations sustains Yablo's core thesis. © 2010 Springer Science+Business Media B.V.