A Syntactical Analysis of Lewis’s Triviality Result

The first part of the paper contains a probabilistic axiomatic extension of the conditional system WV, here named WVPr. This system is extended with the axiom (Pr4): PrA = 1 ⊃ A. The resulting system, named WVPr∗, is proved to be consistent and non-trivial, in the sense that it does not contain the wff (Triv): A ≡ A. Extending WVPr∗ with the so-called Generalized Stalnaker’s Thesis (GST) yields the (first) Lewis’s Triviality Result (LTriv) in the form (♦(A ∧ B) ∧ ♦(A ∧ ¬B)) ⊃ PrB|A = PrB. In §4 it is shown that a consequence of this theorem is the thesis (CT1): ¬A ⊃ (A > B ⊃ A J B). It is then proven that (CT1) subjoined to the conditional system WVPr∗ yields the collapse formula (Triv). The final result is that WVPr∗+(GST) is equivalent to WVPr∗+(Triv). In the last section a discussion is opened about the intuitive and philosophical plausibility of axiom (Pr4) and its role in the derivation of (Triv).