In this chapter, I look at Heidegger's critical response to Husserl's Cartesianism and his effort to articulate a conception of human existence that undercuts the methodological solipsism that provides the general epistemological and metaphysical framework for Husserl's analysis of experience. Heidegger's conception of human existence is intended to undermine the Husserlian assumption that sense and meaning are things that are intelligible in the terms set by methodological solipsism.

The contemporary philosophy of mind has been considerably interested in the merits and demerits of internalist and externalist conceptions of the mental. According to internalists, it is possible to understand at least some contents of the mind in the narrow terms provided by introspection and first-person awareness that does not appeal to any knowledge of other persons or objects outside of the person in question. It is thus possible to investigate such contents of consciousness in a manner that is methodologically solipsistic. Externalists, by contrast, argue that there is no interesting notion of mental content left over once one abstracts from the relations of persons to the public institutions that constitute our linguistic conventions. The externalist argues that these conventions are themselves underwritten by the objects belonging to a shared environment. It is the natural kinds into which objects fall in our environment that give determinate structure to the concepts that persons have.

Heidegger has a wide reputation for his efforts in breaking down the inner–outer distinction of post-Cartesian epistemology and philosophy of mind. Instead of thinking of understanding on the model of a subject that confronts an object, he suggests that we understand human existence as being-in-the-world.