 Idealism. Content externalism is sometimes portrayed as a thesis whose truth precludes idealism in at least some of its major forms (see Rowlands 2003, for example). However, it is not clear that this is really the case. For example, even a Berkeleian idealist could accept Putnam’s twin earth arguments if thus inclined. All that is required for Putnam’s argument to work is a distinction between the way water and twater appear and their underlying structures. This is something the Berkeleian idealist can, it seems, accommodate. For example, the Berkeleian idealist can explain the differences in underlying structure between water and twater in terms of the difference in the ideas that God would flash into your mind and that of your twin post 1750, when you both get down to the business of investigating (what you think of as) the underlying structure of the substance in your oceans and rivers. Content externalism requires a reality that exists independently of the thinking subject or subjects that feature in the thought experiment. But it does not appear to require that this reality be non-mental. Some forms of content externalism require this, supposition for example Sartre (1943 [1957]). But not all forms appear to do so. The relation between content externalism and idealism, in its various forms, is, therefore, not a straightforward one.