Teddy Seidenfeld recently claimed that Kolmogorov's probability theory transgresses the Substitutivity Law. Underscoring the seriousness of Seidenfeld's charge, the author shows that (Popper's version of) the law, to wit:
If (∀D)(Pr(B,D) = Pr(C,D)), then Pr(A,B) = Pr(A,C),
follows from just

five constraints on Pr of the most elementary and most basic sort.