Many other qualifications can be added, making the rules for using a rowboat almost impossible to apply, and yet anyone will still be able to think of additional requirements not yet stated. Namely, the objects that can be shown to have a certain property P by reasoning from certain facts A are all the objects that satisfy P . Thus we circumscribe the set of relevant tuples. Then circumscription may be used to conjecture that the only entities that can prevent the use of the boat are those whose existence follows from the facts at hand. If no lack of oars or other circumstance preventing boat use is deducible, then the boat is concluded to be usable.
Circumscription formalizes several processes of human informal reasoning.
For example, common sense reasoning is ordinarily ready to jump to the conclusion that a tool can be used for its intended purpose unless something prevents its use. Even when a program does not reach its conclusions by manipulating sentences in a formal language, we can often profitably analyze its behavior by considering it to believe certain sentences when it is in certain states, and we can study how these ascribed beliefs change with time. When we do such analyses, we again discover that successful people and programs must jump to such conclusions.