I have distinguished between modal logic and tense logic more than is usual because I hold the distinction to be important. I am speaking of alethic modal logic, and throughout this chapter modal logic otherwise unspecified will mean the alethic variety. The categories of modal logic are not the same as those of tense logic, since the modal categories deal with logical distinctions which are unaffected by time, while the categories of tense logic do not. Modal logic is a branch of formal logic, while tense logic is part of concrete logic, where its treatment in this work will be found.