 The first premise is analytically true; it follows from the way we define theoretical terms. The second premise is something we learn from modern physics. (It isn’t clear, by the way, that we can avoid Smart’s parsimony argument if we really want to argue for premise 2.) So the conclusion is contingent, since modern physics is contingent, but it is well-grounded. Indeed, if we change the second premise a little, drawing on neurology rather than physics, we can draw a stronger conclusion, one that Lewis draws in “Psychophysical and Theoretical Identifications”.