Philosophers typically become acquainted with various types of circularity in the course of their career. Explanations, but more commonly arguments, are called circular when what is to be argued for, or what is to be explained, is being explained or argued for by reference to the very thing.
The classical case is explicit premise circularity: You want to argue for some claim c by saying: suppose c, then c. Anybody who didn’t already agree with c before your argument can reject this argument out of hand. Similarly with explanations: explaining some term T in terms of T itself will be circular.
A less obvious, but still fairly common case is that of rule-circularity. This one is better know in connection with argument. An argument is rule-circular if, in an attempt to justify a certain kind of inference rule (inference to the best explanation being a notorious example), it makes use of that very inference rule.
But is there not a third kind of circularity even harder to detect than rule-circularity: metaphor-circularity? This type is probably more commonly found in explanations. An explanation is metaphor-circular when, in using a metaphor to explain a certain term or phenomenon, the metaphor used itself requires explanation in terms of the term or phenomenon explained.
An example would be this: in the philosophy of action, philosophers sometimes describe their task as “giving an account of the mental economy of a rational agent”. It seems that the idea that there could be a certain kind of account (i.e. one that makes no reference to rationality or agency) of “the mental economy” of an agent is made plausible to no small degree by the use of the term “economy”. After all, we understand what economy is (well, we don’t, but we think we do). So describing human action would be like describing how market forces govern markets.
But wait a minute: don’t we need some conception of what a rational agent (or an idealized rational agent) is, in order to make sense of market behavior. True, we are not explaining market forces in terms of ONE particular agent governing the market, but we nevertheless assume that the participants in a market are rational agents. Market forces are rational forces – producers are forced to buy cheaper materials not because they are held at gunpoint, but because they are rational.
Of course, it will be extremely difficult to prove, for any given case, that it is a case of metaphor-circularity. For one thing, showing that the concepts involved indeed depend on each other for explication is hard. For another, to show that the use made of a given concept falls prey to the conceptual dependence will also not be easy. Lastly, it is not always clear that an explication really depends on the particular metaphor. So let’s be cautious in accusing others of metaphor-circularity, but let’s also be cautious not to accept explanations (including our own) too hastily.