A person's state of mind cannot be adequately regarded as the set of propositions that he believes--at least not if we regard the propositions as sentences that he would give as answers to questions. For example, as we write this we believe that George Bush is the President of the United States, and if we were entering information in a database, we might write
However, my state of mind includes, besides the assertion itself, my reasons for believing it, e.g. he has been referred to as President in today's news, and we regard his death or incapacitation in such a short interval as improbable. The idea of a TMS (see [19]) or reason maintenance system is to keep track of the pedigrees of all the sentences in the database and keep this information in an auxiliary database, usually not in the form of sentences.
Our proposal is to use a database consisting entirely of outer sentences where the pedigree of an inner sentence is an auxiliary parameter of a kind of modal operator surrounding the sentence. Thus we might have the outer sentence
where the dots represent the reasons for believing that Bush is President.
The use of formalized contexts provides a convenient way of realizing this idea. In an outer context, the sentence with reasons is asserted. However, once the system has committed itself to reasoning with the proposition that Bush is President, it enters an inner context with the simpler assertion
If the system then uses the assertion that Bush is President to reach a further conclusion, then when it leaves the inner context, this conclusion needs to acquire a suitable pedigree.
Consider a belief revision system that revises a database of beliefs solely as a function of the new belief being introduced and the old beliefs in the system. Such systems seem inadequate even to take into account the information used by TMS's to revise beliefs. However, it might turn out that such a system used on the outer beliefs might be adequate, because the consequent revision of inner beliefs would take reasons into account.