So, say Chemero and Silberstein in "Defending Extended Cognition" (p. 128).
That seems to be something of an oversimplification. There are those thought experiments in Clark & Chalmers, (1998), i.e., of Inga-Otto and the three modes of Tetris play. Then, Clark (2005, 2008), has a thought experiment about Martians. This thought experiment is also taken up by Menary, (2006). And, Clark (2008, forthcoming) has a "Hippo-world" thought experiment. Thought experiments seem like a priori philosophy of mind to me.
A. Chemero and M. Silberstein, “Defending Extended Cognition”, In Love, McRae, and Sloutsky (eds.), Proceedings of the 30th Annual Meeting of the Cognitive Science Society, 129-134, 2008.
Clark, A. (2005). Intrinsic content, active memory, and the extended mind. Analysis, 65, 1-11.
Clark, A., (2008) Supersizing the mind. Oxford University Press.
Clark, A. (forthcoming). Coupling, constitution and the cognitive kind: A reply to Adams and Aizawa. In Menary, R. The Extended Mind. MIT Press.
Menary, R. (2006) Attacking the bounds of cognition. Philosophical Psychology, 19 329-344.