(The probability associated with a grammar rule may be induced, but the application of that grammar rule within a parse tree and the computation of the probability of the parse tree based on its component rules is a form of deduction.)
This passage describes how the probability associated with a grammar rule is used to determine the likelihood of a particular sentence structure within a larger grammatical context. This process involves applying deductive reasoning to analyze and interpret the relationship between individual rules and the overall structure of a sentence.