The problems we have been discussing naturally give rise to two philosophical points of view (or two philosophical temperaments, as I called them in the Introduction). It is with these points of view, and with their consequences for just about every issue in philosophy that I shall be concerned: the question of ‘Brains in a Vat’ would not be of interest, except as a sort of logical paradox, if it were not for the sharp way in which it brings out the difference between these philosophical perspectives.

One of these perspectives is the perspective of metaphysical realism. On this perspective, the world consists of some fixed totality of mind-independent objects. There is exactly one true and complete description of ‘the way the world is’. Truth involves some sort of correspondence relation between words or thought-signs and external things and sets of things. I shall call this perspective the externalist perspective, because its favorite point of view is a God's Eye point of view.

The perspective I shall defend has no unambiguous name. It is a late arrival in the history of philosophy, and even today it keeps being confused with other points of view of a quite different sort. I shall refer to it as the internalist perspective, because it is characteristic of this view to hold that what objects does the world consist of? is a question that it only makes sense to ask within a theory or description. Many ‘internalist’ philosophers, though not all, hold further that there is more than one ‘true’ theory or description of the world.