Justification, according to the tripartite theory of knowledge, is the difference between merely believing something that is true, and knowing it. To have knowledge, on this account, we must have justification. How our beliefs are justified is among the central questions of epistemology.
Having justification for our beliefs is, plausibly, about having good reasons to think that they are true. For a belief to be justified, it seems, it must be inferred from another belief. This type of justification is called inferential justification. It seems that three conditions must be met for a belief to be inferentially justified.
First, there must be some other idea that supports it. This other idea need not establish what is believed with absolute certainty, but it must lend some degree of support to it, it must render the belief probable. Without a supporting idea, there can be no inferential justification.
Second, we must believe that this other idea is true. It is not enough for justification that there be another idea that supports our belief; if we thought that that other idea were false then it could not possibly help to justify our belief. Inferential justification, therefore, requires the existence of a supporting idea that is believed to be true.
Third, we must have good reason for believing that this supporting idea is true. If we irrationally believe the supporting idea, then that irrationality will transfer to the belief that we base upon it; a belief can only be as justified as are the other beliefs on which it is based. For a belief to be inferentially justified, therefore it must be based in a supporting idea that is believed to be true with justification.