 Burley deals with these topics for the first time in his juvenile middle commentary on the Categories and Tractatus de abstractis (still unedited). In former work, he claims that substantial essences have no being (esse) outside the being of their instantiations (TsP, ch. de substantia, ed. Conti, pp. 34–35). Individual substances are unique metaphysical entities, located at a particular place and time, and substantial essences are their specific forms, apt to be common to several singulars at the same time, present in them, and having no independent existence. An individual substance, such as the substance of Socrates, is compounded not only by singular form and matter, but also by any substantial essence which is over-ordered to the singular substance and therefore predicated in quid of it. Furthermore, in his opinion, singular substances can be acknowledged as members of a given (natural) species by virtue of their conformity to the abstract essence they instantiate (TsP, ch. de substantia, ed. Conti, pp. 36–37; see also pp. 32–33). 