The first five chapters deal with the terms that form propositions. Chapters 6 and 7 deal with the relationship between affirmative, negative, universal and particular propositions. These relationships are the basis of the well-known Square of opposition. The distinction between universal and particular propositions is the basis of modern quantification theory. The last three chapters deal with modalities. Chapter 9 is famous for the discussion of the sea-battle. (If it is true that there will be a sea-battle tomorrow, then it is true "today" that there will be a sea-battle. Thus a sea-battle is apparently unavoidable, and thus necessary. Another "interpretation" would be: that we cannot know that which has not yet come to pass. In other words: if there is a sea battle tomorrow then it is true today that tomorrow there will be a sea battle. So, only if we can know whether or not there will be a sea battle tomorrow then can we know if there will be a sea battle).