A few years ago I had occasion to visit Peru, and I got to know a fine philosopher and a truly wonderful human being-Francisco Miro Casada. Miro Casa… - Hilary Putnam

Part of what makes moral philosophy an anachronistic field is that its practitioners continue to argue in this very traditional and aprioristic way even though they themselves do not claim that one can provide a systematic and indubitable 'foundation' for the subject. Most of them rely on what are supposed to be 'intuitions' without claiming that those intuitions deliver uncontroversial ethical premises, on the one hand, or that they have an ontological or epistemological explanation of the reliability of those intuitions, on the other.

Before I give the argument, let us consider why it seems so strange that such an argument can be given (at least to philosophers who subscribe to a 'copy' conception of truth). We conceded that it is compatible with physical law that there should be a world in which all sentient beings are brains in a vat. As philosophers say, there is a 'possible world' in which all sentient beings are brains in a vat. (This 'possible world' talk makes it sound as if there is a place where any absurd supposition is true, which is why it can be very misleading in philosophy.) The humans in that possible world have exactly the same experiences that we do. They think the same thoughts we do (at least, the same words, images, thought-forms, etc., go through their minds). Yet, I am claiming that there is an argument we can give that shows we are not brains in a vat. How can there be? And why couldn't the people in the possible world who really are brains in a vat give it too?
The answer is going to be (basically) this: although the people in that possible world can think and 'say' any words we can think and say, they cannot (I claim) refer to what we can refer to. In particular, they cannot think or say that they are brains in a vat {even by thinking 'we are brains in a vat').

To require that all of these must be reducible to a single version is to make the mistake of supposing that 'Which are the real objects?' is a question that makes sense independently of our choice of concepts.