This paper addresses the question whether introspection plus externalism about mental content warrant an a priori refutation of external-world skepticism and ontological solipsism. The suggestion is that if thought content is partly determined by affairs in the environment and if we can have non-empirical knowledge of our current thought contents, we can, just by reflection, know about the world around us -- we can know that our environment is populated with content-determining entities. After examining this type of transcendental argument and (...) discussing various objections found in the literature, I argue that the notion of privileged self-knowledge underlying this argument presupposes that we can learn, via introspection, that our so-called thoughts are propositional attitudes rather than contentless states. If, however, externalism is correct and thought content consists in the systematic dependency of internal states on relational properties, we cannot know non-empirically whether or not we have propositional attitudes. Self-knowledge (apropositional attitude) is consistent with us lacking the ability to rule out, via introspection, the possibility that we don't have any propositional attitudes. Self-knowledge provides us with knowledge of what is in our minds, but not that we have minds. Hence, the combination of externalism with the doctrine of privileged self-knowledge does not allow for an a priori refutation of skepticism and is therefore unproblematic. (shrink)