 When this strategy is followed, and leaving technical details aside, the most important decision is the possible introduction of bridge principles that link the main modalities of the systems to be combined. Paraphrasing Schurz (1991), a schema \(\varphi\) is a bridge principle if and only if it contains at least one schematic letter which has at least one occurrence within the scope of the modality of one system and at least one occurrence within the scope of the modality of the other. (This definition was given in the context of the David Hume’s discussion on whether ought can be derived from is; see  Section 1  of  combining logics.)