Some beliefs are supported (justified) by other beliefs-I may believe A because I believe B. These other beliefs may themselves be supported by still further beliefsperhaps I believe B because of C-and so on. For a given belief or piece of knowledge, then, we can ask what is the overall structure of the series of reasons for itthe structure of its justification. There are four ultimate possibilities (see Figure 7.1):

(a) There could be a finite series of reasons, none of which repeat. In this case, I come finally to some belief for which I have no further reasons.