Because philosophy operates at a presuppositional level by clarifying and justifying the presuppositions of a discipline, philosophy is the only field of study that has no unquestioned assumptions within its own domain. In other words, philosophy is a self-referential discipline, for questions about the definition, justification and methodology of philosophy are themselves philosophical in nature. Philosophers keep the books on everyone, including themselves.

J. P. Moreland and William Lane Craig, Philosophical Foundations for a Christian Worldview, Illinois, 2003, p. 13