 Some medieval works fall somewhere between aphorism and axiom in genre, most significantly works connected to the important and influential Liber de causis. The Liber circulated as a work of Aristotle under the title Liber Aristotelis de expositione bonitatis purae until the 13th century, when Thomas Aquinas found its source in Proclus’s Elements of Theology. Like Proclus’s theology, the work seems to present its principles as axioms, but the principles and their explanation/derivation is not really demonstrative and the principles themselves are highly abstract Neoplatonic metaphysical principles that are sometimes as paradoxical as they are self-evident. Boethius’s theological opuscule known as De Hebdomadibus (this work is traditionally the third of five theological tractates) and Alan of Lille’s Regulae Caelestis Iuris are presented as axiomatic but also esoteric, with the express statement of the author that their principles are not accessible to the many. (See section 2.3 below, on axiom, for more discussion of these works.)