 Now, Prior introduced hybrid tense logic in connection with what he called four grades of tense-logical involvement. The motivation for his four grades of tense-logical involvement was philosophical. The four grades were presented in the book Prior (1968), Chapter XI (also Chapter XI in the new edition Prior (2003)). Moreover, see Prior (1967), Chapter V.6 and Appendix B.3-4. For a more general discussion, see the posthumously published book Prior and Fine (1977). The stages progress from what can be regarded as pure first-order earlier-later logic to what can be regarded as pure tense logic; the goal being to be able to consider the tense logic of the fourth stage as encompassing the earlier-later logic of the first stage. In other words, the goal was to be able to translate the first-order logic of the earlier-later relation into tense logic. It was with this goal in mind Prior introduced so-called instant-propositions: