A major role in Bolzano's logical theory is played by the notion of "variations": various logical relations are defined in terms of the changes in truth value that propositions incur when their non-logical parts are replaced by others. Logically analytical propositions, for instance, are those in which all the non-logical parts can be replaced without change of truth value. Two propositions are 'compatible' ("verträglich") with respect to one of their component parts x if there is at least one term that can be inserted that would make both true. A proposition Q is 'deducible' ("ableitbar") from a proposition P, with respect to certain of their non-logical parts, if any replacement of those parts that makes P true also makes Q true. If a proposition is deducible from another with respect to all its non-logical parts, it is said to be 'logically deducible'.