Beliefs, desires, intentions, and many other mental attitudes have a subjective character. We are able to, for instance, subjectively have belief, thought, desire, and intention even concerning things that do not exist. That we can to such things helps us to understand the internalistic intuition that contents of mental attitudes are determined solely internally, that is, independently of the external world. The traditional view that mental contents are internally determined, however, is today strongly challenged by the view of mental content externalism sparked by Tyler Burge. By applying H. Putnam’s argument for meaning externalism, he argues that not only meaning but also mental content necessarily depend on causal and social relations of the external world. This article critically examines this argument. Specifically, it analyzes the type of argument presented by Burge for externalism and argues that it fails to be convincing enough to establish the view. To this end, I will first look into the way the main argument for mental content externalism is given and reveal that the view is ultimately based on semantic reason. Then, I will explore whether linguistic descriptions of mental contents are necessary or sufficient to understand and determine them. It will be shown from the discussion that linguistic descriptions are neither necessary nor sufficient for it. And this will lead to the conclusion that the semantic reason presented by Burge for mental content externalism does not lend proper support to it in the end.