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The derivation of Bayes' theorem requires that the conditioning probabilities be nonzero. Specifically, when deriving $P(A|B)$ from the intersection, we divide by $P(B)$, so $P(B)$ must be positive.

Conditions: Deriving Bayes' formula from conditional probability definitions.; $P(B) > 0$ for $P(A|B)$ to be defined.; $P(A) > 0$ for $P(B|A)$ to be defined.

The derivation of Bayes' theorem requires that the conditioning probabilities be nonzero. Specifically, when deriving $P(A|B)$ from the intersection, we divide by $P(B)$, so $P(B)$ must be positive.

Conditions: Deriving Bayes' formula from conditional probability definitions.; $P(B) > 0$ for $P(A|B)$ to be defined.; $P(A) > 0$ for $P(B|A)$ to be defined.