A&B cannot be more probable than A, but evidence may support A&B more than it supports A.
For example, suppose you have independent prior probabilities of 1⁄2 for A and for B. The prior probability of A&B is 1⁄4. If you are then told “A iff B,” the probability for A does not change but the probability of A&B goes up to 1⁄2.
The reason specific theories are better is not that they are more plausible, but that they contain more useful information.
A&B cannot be more probable than A, but evidence may support A&B more than it supports A.
For example, suppose you have independent prior probabilities of 1⁄2 for A and for B. The prior probability of A&B is 1⁄4. If you are then told “A iff B,” the probability for A does not change but the probability of A&B goes up to 1⁄2.
The reason specific theories are better is not that they are more plausible, but that they contain more useful information.