Notes on Hempel's Dilemma

Hempel’s dilemma purports to present an intractable problem for physicalism:
(H1) If physical properties are by definition the properties expressed by the predicates of a current physical theory, physicalism is false. (H2) If physical properties are by definition the properties expressed by the predicates of an ideal physical theory, we don’t know what physicalism says. (H3) Either it is the case that physical properties are by definition the properties expressed by predicates of a current physical theory, or it is the case that the physical properties are by definition the properties expressed by predicates of an ideal physical theory. (HC) Either physicalism is false or we don’t know what it says.[1]
The first premise is about properties. A property, on physicalism, is either a physical property or necessitated by a physical property. However, a legitimate property is one that is necessitated by a physical property, that is a property legitimated by current physics. Current physics is incomplete; that is, current physics does not claim to have a complete inventory of properties. However, if physics is incomplete then any future found property is impossible since by definition it is neither physical nor necessitated by a physical property since it is not by definition necessitated by current physics.

The second premise says that even if we name properties of a future, complete physics we cannot know what they are. Contending theories for a complete theory might be true, but we cannot know which one nor whether a yet unknown theory is true.

Carl Gustav Hempel

Some suggest we should not worry about whether or not a current theory is true or not. It is the best we have and it appears to work for the time being. Moreover, at least part of our current theory is going to be true and that's enough to be going on with.

The third premise is where Stoljar thinks there is a problem. It appears to be a logical truth (A v B). Stoljar argues that it is neither a logical truth nor a truth at all. It is not a necessary truth that physical properties be defined in terms of either a current or a future physical theory. There is not just one theory, but many. Since the premise is a disjunction it is false if both side of the disjuncts are false. Neither a current physics nor a future physics exists. There is not one current physics, but many and there is no future physics. Therefore the disjunct is false.

[1] Daniel Stoljar, Physicalism (New York: Routledge, 2010), 97-98.

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