Why was Finbar really away from school for so long?

Finbar constructed of a set elaborate lies and slander which consist entirely of fabricated statements about the absense matter under consideration. However, the truth of any one of these statements is always relative to the whole theory of deciept. Therefore the same statement may be true with respect to one theory, and not true with respect to another. This is, in ordinary language, where statements such as "He is a terrible person" cannot be judged to be true or false without reference to some interpreation of who "such a monstrousity" is and for that matter what a "terrible person" is under the theory.

Sometimes two theories of Finbars omission from school have exactly the same explanatory power because they make the same predictions, he wasn't there. A pair of such theories is called indistinguishable, and the choice between them reduces to convenience or philosophical preference.