Gilbert of Poitiers (Gilbert de la Porrée, 1085?–1154 AD) in different commentaries to Boethius’s De Trinitate, De Hebdomadibus, and Contra Eutychen et Nestorium proposes an original theory of individuals, according to which:

• everything that exists is necessarily particular (individual);
• an individual (subsistens) is a subsisting entity;
• an essential property (subsistentia), is a particular essential property (or form) determining the being of just one individual;
• each individual (subsistens) is an ordered bundling of properties, which Gilbert calls concretio, the set of all the properties that might be true of a given subject is the total form

Gilbert’s model of individuals is presented in the following OntoUML diagram:

Class	Description	Relations
Individual	“An id quod est is an independent individual, such as Peter, my cat Felix, or this apple. Gilbert uses an alternative terminology: an id quod est, an individual, is also called a subsistens, a subsisting entity. […] An individual is the sum, the collection of all its essential subsistentiae and particular accidental properties.”
TotalForm	“Gilbert offers a very interesting theoretical tool for the consideration of an individual seen over a whole lifetime. He probably pursues the doctrinal aim of ensuring personal identity across time and of avoiding that an individual undergoing qualitative or quantitative change becomes a different individual. Gilbert introduces the concept of forma tota [total form] to refer to the set of all the properties that might be true of a given subject, including also those that will never be actualized. The sum of the essential properties and accidents thus constitutes the forma tota of the individual; the sum of the characteristics of Socrates is called Socrateitas and the sum of those of Plato, Platonitas. According to Gilbert, the tota forma of an individual—for example, Plato’s Platonitas— includes all its subsistences (its substantia tota) but also its quantitative and qualitative accidents—not only those it possesses actually (actu), but also those it possessed in the past and those it will or could possess in future according to the potentiality of its own nature. Expressed as bundle, the forma tota could be understood as the most comprehensive possible bundle. This total form belongs to just one concrete being. The individuality of a concrete being or, in Gilbert’s words, its numerical diversity, is caused by the particularity of its essential properties and rendered manifest by the fact that it possesses its own total form. An anonymous Porretan treatise is explicit when it speaks of the case of the individual Peter. The form which is composed of all the various properties, both essential and accidental, which are in Peter, and which make Peter different from all the other men, is called petritas. This form is particular, and explains how Peter is a being discrete from all other men.”	characterizes Individual
Essential Property	“An id quo —that is, an essential property—is also called subsistentia, a particular essential property or substantial form determining the being of just one individual. Each individual, or subsistens, is constituted of several subsistentiae. The interest and originality of Gilbert’s position lie in the fact that each of these subsistentiae is particular.” E.g. The animality of Felix, the cat.	component of Essence; inherits from Property
Essence	“All these subsistentiae taken together constitute the essence of an individual, which Gilbert calls tota substantia.“	component of TotalForm
Accidental Property	“Gilbert divides accidental properties into two groups: accidents proper (that is, quantitative and qualitative properties) and extrinsic properties that he calls, following Boethius, ‘circumstances‘ (rei circumstantiae)”	inherits from Property
AccidentProper	“The relation between accidents [proper] and essential properties is that of foundational relatedness; an accident must be attached to an essential subsistentia. Gilbert insists on the fact that, in order to be a part of the bundle of properties which the individual or subject is, an accident must first be attached to an essential property: ‘For colour adheres (adest) to corporality in order that it might inhere in a body.’ So an accident adheres to a subsistentia; but an accident cannot adhere to just any subsistence; it must adhere to the subsistence which causes it, for it is the only one which can cause it. […] The real accidents inhere in the concretion (intrinsecus concretionis habitus)“ E.g. the blackness of the fur of Felix, the cat.	component of TotalForm; inherits from Accidental Property
Circumstantial Property	“the circumstances [circumstantial properties] are just apposed (extrinsecus cuiuslibet appositionis habitus).” E.g. The location on the fence of Felix the cat on a given moment.	component of TotalForm; inherits from Accidental Property
Property	“We have seen that these properties are all particular.”

*I added the attribute validFromTo to mark the temporal validity of the Essential Property, Accident Proper, and Circumstantial Property. E.g. for Accident Proper: The weight of Felix the cat is 4 Kg from 04/07/2020 to 14/08/2020.

Sources

• All citations from: Erismann, Christophe, “Explaining Exact Resemblance: Gilbert of Poitiers’s Conformitas Theory Reconsidered”, Oxford Studies in Medieval Philosophy, Volume 2, Edited by ROBERT PASNAU,2014
• Bieniak, Magdalena: “Individuals as Wholes. Gilbert of Poitiers’s Theory of Individuality“, Mereology in Medieval Logic and Metaphysics, Proceedings of the 21st European Symposium of Medieval Logic and Semantics, edited by Fabrizio Amerini, Irene Binini, Massimo Mugnai, 2019
• Adamson, Peter, “216. One of a Kind: Gilbert of Poitiers on Individuation“, History of Philosophy without any Gaps podcast

First published: 04/07/2020