We present a framework for reasoning about the concepts of "knowing what" and "finding out", in which the key concept is to identify "finding out the answer to question Q" with "achieving a situation in which Q is decidable" . We give examples of how the framework can be used to formulate non-trivial problems involving the construction of plans to acquire and use information, and go on to demonstrate that these problems can often be solved by systematic application of a small set of goal-directed backward-chaining rules. In conclusion, it is suggested that systems of this kind are potentially implementable in l-Prolog, a logic programming language based on higher-order logic.