Wimsatt and Beardsley divide the evidence used in making interpretations of literary texts (although their analysis can be applied equally well to any type of art) into three categories:
(1) Internal evidence. This evidence is present as the facts of a given work. The apparent content of a work is the internal evidence, including any historical knowledge and past expertise or experience with the kind of art being interpreted: its forms and traditions. The form of epic poetry, the meter, quotations etc. are internal to the work. This information is internal to the type (or genre) of art that is being examined. Obviously, this also includes those things physically present to the work itself.
(2) External evidence. What is not actually contained in the work itself is external. Statements made privately or published in journals about the work, or in conversations, e-mail, etc. External evidence is concerned with claims about why the artist made the work: reasons external to the fact of the work in itself. Evidence of this type is directly concerned with what the artist may have intended to do even or especially when it is not apparent from the work itself.
(3) Contextual evidence. The third kind of evidence concerns any meanings derived from the specific work's relationship to other art made by this particular artist—as is the way it is exhibited, where, when and by whom. It can be biographical, but does not necessarily mean it is a matter of intentional fallacy. The character of a work may be inflected based upon the particulars of who does the work without necessarily characterizing it as an intentional fallacy.
Thus, a text's internal evidence — the words themselves, and their meanings — is fair game for literary analysis. External evidence — anything not contained within the text itself, such as information about the poet's life — belongs to literary biography, not literary criticism. Preoccupation with the author "leads away from the poem." According to New Criticism, a poem does not belong to its author, but rather "it is detached from the author at birth and goes about the world beyond his power to intend about it or control it. The poem belongs to the public." It is the Contextual evidence that presents the greatest potential for intentional fallacies of interpretation. Analysis using this type of evidence can easily become more concerned with external evidence than the internal content of the work.