One of the chief aims of Donald Davidson's later work was to show that participation in a certain causal nexus involving two creatures and a shared environment-Davidson calls this nexus "triangulation"-is a metaphysically necessary condition for the acquisition of thought. This doctrine, I suggest, is aptly regarded as a form of what I call transcendental externalism. I extract two arguments for the transcendental-externalist doctrine from Davidson's writings, and argue that neither succeeds. A central interpretive claim is that the arguments are primarily funded by a particular conception of the nature of non-human animal life. This conception turns out to be insupportable. The failure of Davidson's arguments presses the question of whether we could ever hope to arrive at far-reaching claims about the conditions for thought if we deny, as does Davidson, the legitimacy of the naturalistic project in the philosophy of mind.