This definition requires some explanation.  Clause (a) distinguishes memory belief from beliefs newly formed on the basis of a memory image. If one has never before believed p, then S can't now have a memory belief that p, although one might now form a belief for the first time on the basis of an event memory. Clause (b) is necessary to distinguish cases in which one believes that P at one time, completely and utterly forgets P, and then relearns P from an independent source. Clause (c) requires that the belief not be occurrent all the time from the time it was formed until the time at which we are calling it a memory belief.  I'm not certain this restriction is required, but I think having it rules out certain complicating factors.  For example, if I believe that P and hold it in occurrent, working memory for one minute, say, shall we now count it as a memory belief?  What if I hold it in memory for ten seconds?  Two seconds?  If we require that a belief must have been nonoccurrent (while still continuing to exist) for a time before it's now counting as a memory belief, we don't have to deal with these issues, which are anyway not central to our concerns.  One other important note here: I have required that it be the “same belief” that is at one time occurrent, then dispositional, and then occurent again.  This is not the requirement that it be the same token belief all along, but only a belief with the same content that, after its first appearance, is appropriately causally related to earlier tokens of that person's having that belief with that content.  When I believe that my car is locked because I just checked the doors, and later recall that my car is locked, it might be that my later belief is not token identical with the earlier belief.  On the other hand, I don't want to insist that it is not the same token.  I shall be leaving this issue open.