Nicholas and Thomas consider the philosophical background of the two doctrines. The Stoic view gave primacy to matter, whilst Aristotle considered that form or essence was what mattered most. Nicholas considers that the Aristotelian 'form' or 'essence' faces the same difficulty of definition as 'identity'. Andrew Borkowski considers that the Proculians treated specificatio as a form of occupation. At the moment of its creation, a nova species was regarded as a res nullius open to the first occupier, and for that reason it should be held to belong to the creator.