OP was talking about priors, which is the part before we look at the content. OP is saying, I think, that the prior should be the fraction of all possible worlds in which the proposition is true.
Whether you accept this example depends on how you interpret the phrase “possible worlds”. If “all possible worlds” is all possible values of a binary string, and “possible worlds in which P is true” is all binary strings in which position P = 1, then A or B or C gets a higher prior than A and B and C.
However, if you compute your “prior” not when you are a Cartesian blank slate with no evolutionary history, but when you already know something about the world, then “all possible worlds” could mean “all possible values of A, B, and C that are consistent with previous information”. In that case, I would expect that “A and B and C” and “A or B or C” would have more similar priors, provided that the set {A, B, C} was chosen deliberately. A person reasoning about A, B, and C probably chose them due to some perceived commonality, meaning there is some underlying rule about things in this set, and the hypothesis being tested is really “is this an ‘OR’ rule or an ‘AND’ rule?”
OP was talking about priors, which is the part before we look at the content. OP is saying, I think, that the prior should be the fraction of all possible worlds in which the proposition is true.
Whether you accept this example depends on how you interpret the phrase “possible worlds”. If “all possible worlds” is all possible values of a binary string, and “possible worlds in which P is true” is all binary strings in which position P = 1, then A or B or C gets a higher prior than A and B and C.
However, if you compute your “prior” not when you are a Cartesian blank slate with no evolutionary history, but when you already know something about the world, then “all possible worlds” could mean “all possible values of A, B, and C that are consistent with previous information”. In that case, I would expect that “A and B and C” and “A or B or C” would have more similar priors, provided that the set {A, B, C} was chosen deliberately. A person reasoning about A, B, and C probably chose them due to some perceived commonality, meaning there is some underlying rule about things in this set, and the hypothesis being tested is really “is this an ‘OR’ rule or an ‘AND’ rule?”