This work was surely written by Gundissalinus, as many textual and historical data show. For unknown reasons, however, De unitate had a pseudo-epigraphical circulation since the early 13th century. Most philosophers (Aquinas aside) thought that it was an original work by Boethius – a much consequential attribution! As a result, De unitate was read and used as authoritative source throughout the Middle Ages, up to Francisco Suárez.

Translated by Nicola Polloni, “Dominicus Gundissalinus’s On Unity and the One”, in Contextualizing Premodern Philosophy: Explorations of the Greek, Hebrew, Arabic, and Latin Traditions, ed. by L.X. López Farjeat, K. Krause, and N. Oschman (London: Routledge, 2023), 293-308, on 299-304.

Unity is that by which each thing is said to be one. Whether it is simple or composite, spiritual or corporeal, a thing is one by unity. It can be one only by unity, just as it can be white only by whiteness, or be so much only by quantity. Besides, [a thing] is not only one by unity, but as long as it is something, it is what it is as long as unity is in it. And when it ceases to be one, it ceases to be what it is. For this reason, it has been said that “whatever exists, therefore, exists because it is one,” which is demonstrated as follows.

Undoubtedly, in created things, all existence comes from the form. However, existence comes from the form only when the form is made one with matter. In fact, there is existence only by the joining together of the form with matter. For this reason, the philosophers describe [matter] by saying that “existence is the presence of form in matter.”

However, when the form is made one with matter, something which is “one” necessarily comes to be from their joining together. And that thing, in its coming to be, only persists as long as unity holds the form together with matter. As a consequence, the destruction of a thing is nothing else but the separation of [its] form from matter. Separation and union are contraries, though. Therefore, if something is destroyed by the separation [of form and matter], that thing is surely preserved in its existence by [their] union.

Union [unitio], however, only exists by unity [unitas]. When unity is separated from something united, its union, by which that thing was one, is dissolved. And when the union is dissolved, the essence of that thing— which stemmed from the union [of matter and form]—is destroyed, because it becomes something which is not one. For this reason, not only is a thing brought to existence by unity but existence is also maintained in that thing by unity. Therefore, existence and one inseparably accompany each other and appear to exist together in nature.

Since the Creator is the true One, the things he established—each of them—received [its] existence one as a gift from Him. As a consequence, anything which receives its existence from Him is one. Accordingly, every substance moves toward and through the One, and none of the existing things desires to be many. To the contrary, desiring to exist, all of them desire to be one, since everything desires by nature to exist, and it can exist only by being one. Therefore, everything tends to [be] one. Unity, indeed, is what makes everything one and holds everything together, for it is diffused into every existing thing.

On this account, considering that matter has existence only by the union with its form and that only unity can keep the form united to matter, matter requires unity in order to become one in itself and acquire existence. Matter, indeed, is contrary to unity. It is so because matter, by itself, flows away and its own nature is to be multiplied, divided, and dispersed, whereas unity holds, unites, and keeps it together. For this reason, matter must be held together by unity in order not to divide or disperse itself. In fact, anything requiring something else to become one cannot become one by itself. Nonetheless, what cannot become one by itself certainly is dispersed by itself. Indeed, anything that is able to make something contrary to a [considered] agent makes it contrary to what [that agent] has made. In fact, contraries are the effects of what is contrary. Since unity causes [something to be] one; therefore, matter will cause division. Accordingly, unity by itself holds matter together. And whatever holds [something] together by itself cannot be the cause of [its] separation. Therefore, the form existing in matter, which completes and holds together the essence of everything, is the unity descending from the first Unity that created it.

In fact, the first and true Unity, which is in itself Unity, created another unity, which lies below it. Yet since every created thing must be completely different from what has created it, the created unity must be completely different from and almost opposite to the creating Unity. Because the creating Unity has neither beginning nor end, neither change nor diversity; therefore, multiplicity, diversity, and mutability accrue to the created unity. In some matter, then, [unity] has a beginning and an end, while in another [matter] it has a beginning, but not an end, because in some it is subject to change and corruption. And in others, [unity is subject to] change, but not to corruption. In those things [in which] matter is subtle, simple, far from contrariety and separation, unity is indeed proportionate to it, and [is] made one with it in such a way that both become [something which is] one, indivisible in act. This is the case for the celestial bodies, in which unity is inseparable from matter.Accordingly, they have no end, for they are perpetual. However, in those things [in which] matter is thick [and] weak, unity cannot be proportionate to it. Indeed, its unifying power and [its capacity to] hold their essence together is weakened. As a consequence, their essence is dissolved because they are not held together by unity. This is the case of generated things, which have a beginning and an end. For the closer any unity is to the first and true Unity, the more one and the simpler will be the matter it informs. And to the opposite, the further unity is from the first Unity, the more multiplied and composed [its matter will be].

Accordingly, the unity that brings the matter of the Intellect to existence is more “one” and simpler, not multiplied or divisible by essence. And if it is divisible, it will be so by accident. This unity is more “one” and simpler than any other unity that brings the other substances to existence, for it is joined without mediation to the first Unity that created it.

However, since the unity subsisting in the matter of the Intellect is the unity of simplicity, the unity subsisting in the matter of the Soul, which is below it, necessarily grows and multiplies [itself ]. As a consequence, change and diversity happen to it. Unity, then, is expanded and multiplied little by little while descending from what is superior through every degree of the inferior matter, until it reaches the substance which bears quantity, that is, the substance of this world. Being furthest from the first Unity, [this matter] is thick, bodily, and compact, and due to its thickness and largeness, it is opposed to the superior substance, which is subtle and simple. In fact, the latter is the subject of the onset and the beginning of unity, while the former is the subject of the end and the extremity of unity. The end, however, is very far from the beginning, since it is only called “end” insofar as it is a failure of power and a limit. The degradation of simplicity and the diminution of its power happen through the descent of the unity from the higher to the lower. This is similar to the water that is subtle and clear in its source but, flowing down little by little, becomes thick and dark in marshes and ponds. In a similar fashion, unity varies little by little through the varieties of the matter bearing it. In fact, since something of matter is spiritual while something else is corporeal, [in it] something is pure and bright, while something else is thick and dark. And this happens because of quantity, whose parts are more dispersed in some things, such as the air, and more compact in other things, such as a stone. Following the degrees of its distance from the first Unity at the origin, each and every part of matter receives unity, which is nobler [than matter] in reason of its property. Accordingly, we see the parts of fire as “one” in every way, simple and equal, so that its shape appears to be one, having no diversity in itself. To the contrary, we find the parts of air and water to be more diverse and separate, so that it is possible to distinguish among their parts and unities. In hard and thick things, however, diversity and darkness are already greater [than that].

In the highest things, matter is informed by the form of the Intellect, and further on, by the form of the Rational Soul, while afterward by the form of the Sensible Soul. Then, below that, [it is informed] by the form of the Vegetative Soul, and after that, by the form of Nature. And finally, in the lowest things, [matter is joined] to the form of the body. All this does not happen because of the diversity of the power of the agent, but because of the property of the matter receiving it.

Form, indeed, is like light. For just as a thing is seen on account of light, so too cognition and knowledge of things are provided by form, and not by matter. This light, however, is brighter in some things and darker in others, depending on whether the matter in which it is infused comes to be brighter or darker. The more sublime matter is, the subtler it will be, and completely permeated by light. Consequently, that substance will be wiser and more perfect, such as the Intelligence and the Rational Soul. And on the contrary, the lower matter is, the thicker and darker it will be, not completely permeated by light. As has been said already, the more matter descends, [the more] it is made compact, thick, and bodily, and its middle parts block the last ones from being perfectly permeated by light. In fact, it is impossible for light to permeate the second part as much as [it does] the first, nor does as much light reach the third part as reaches the second part, and so on, little by little, down to the lowest part, in which the light is weakened. For it is furthest away from the source of light.

Nonetheless, as it has been said, this does not happen on account of the light in itself, but on account of the great density and obscurity of matter in itself. Similarly, when the sunlight is mixed with the dark air, it lacks the power [that it has] when is mixed with bright air. And similarly, the whiteness of a very thin white cloth is occluded by the abundance of blackness when it is worn by a black body. And similarly, if three or more glass windows are set up in order one after another perpendicularly to the sunlight, it is surely ascertained that the second [window] receives less light than the first, and the third less than the second. And up to the last one, there is a diminution of light which is due not to the light itself, but to the distance of the glass windows from the light. In the same way, the light of the form of unity which is infused into matter becomes weak and dark while descending, as [when] the light which passes through the first of these [windows] is different from [that passing through] the second one, and [that passing through] the second [is different] from [that passing through] the last one.

Because of this difference in the form of unity, something is said to be one by unity, not in one, but in many ways. For there is something that is [said to be] one by the simplicity of essence, namely God. And another is [said to be] one by the conjunction of simples, namely, angels and souls, each of which is one by the conjunction of matter and form. And another is [said to be] one by continuity, such as a tree or a rock. And another is [said to be] one by composition, as one ark [is made] of many planks or a house of many spaces. And other things are said to be one by aggregation, such as a people or a flock, a jumble of stones, or a heap of wheat. Others are said [to be] one by analogy, such as when the helmsman of a ship and the governor of a town are said [to be] one by the similarity of their office. Other things are said to be one by accident, as different subjects of the same quality are said [to be] one in that [quality], such as that the snow and the swan are one in their whiteness. Others are said [to be] one by number, as different accidents inhering in the same subject are said to be one by number—that is, by counting, such as [when we say that] this [is] sweet and this [is] cerulean, or this [is] long and this [is] wide. Other things are said [to be] one by reason, but [they are] so in two ways: by reason of a common possession, as the intellect, the [intellected] thing, and [its] word are one in genus; and by reason of one sacrament, as spirit, water, and blood are said [to be] one. Other things are said to be one by nature, as many humans are one by their participation in the species. Others are said [to be] one in virtue of [their] nation or language, as many humans are said [to be] one people or one tribe. Other things are said [to be] one by habit, yet in two ways. Indeed, [many humans are so] by the agreement of virtue and love, as [when it has been said that] “the multitude of believers was one heart and one soul.” However, many humans are said [to be] one [also] by assent to the same vices, as [when it has been said that] “who joins a prostitute becomes one [in the] body.”

In this way, everything desires unity, and it is also said that what is multiple wants to be one. In fact, whatever exists is what it is either because it strives to be a real unity or because, at least, it strives for that by imitating [it].

Every existing thing is one or many. Nonetheless, plurality only exists by the aggregation of unities, which become a multitude when they are dispersed and a magnitude when they are continuous in matter. As a consequence, there is no difference between the unities [composing] a discrete quantity and those [composing] a continuous quantity subsisting in matter, except that the former are dispersed while the latter are continuous. Therefore, what is continuous comes forth only from what is dispersed, since the meaning of continuity in what is continuous is just the continuation of the dispersed [unities]. Accordingly, continuous quantity necessarily comes forth into substances only through unities.

Whatever part of quantity one might choose must necessarily be one or many. As it has been said, however, every plurality derives from unities. Whence it is clearly understandable that discrete and continuous quantities have one root, since they are composed from one thing and are resolved into one [thing]. And [it is clear], too, that the more connected and compacted the parts of a body are, the thicker and more “quantum” [magis quantum] that body will be, such as in the case of a stone. Whereas to the opposite, the more dispersed and scattered the parts of a body are, the subtler, lighter, and less “quantum” [minus quantum] it will be, such as in the case of the air. As a consequence, it is true that continuous quantity comes into substance only on account of unity joining and flowing in it.

Unity, therefore, is that by which each thing is one, and [that by which that thing] is what it is.

Ed. María Jesús Soto Bruna and Concepción Alonso Del Real, De unitate et uno de Dominicus Gundissalinus (Pamplona: EUNSA, 2015).

Vnitas est qua unaquaeque res dicitur esse una. Siue enim sit simplex siue composita, siue spiritualis siue corporea, res unitate una est. Nec potest esse una nisi unitate, sicut nec alba nisi albedine, nec quanta  nisi quantitate. Non solum autem unitate una est, sed etiam tamdiu est  quidquid est id quod est quamdiu in se unitas est. Cum autem desinit esse unum, desinit esse id quod est. Unde est illud: ‘quidquid est, ideo est, quia unum est’. Quod sic ostenditur:

Omne enim esse ex forma est, in creatis scilicet. Sed nullum esse ex forma est nisi cum forma materiae unita est. Esse igitur non est nisi ex coniunctione formae cum materia. Vnde philosophi sic describunt illud dicentes: ‘esse est existentia formae in materia’.

Cum autem forma materiae unitur, ex coniunctione utriusque necessario aliquid unum constituitur. In qua constitutione illud unum non permanet nisi quamdiu unitas formam cum materia tenet. Igitur destructio rei non est aliud quam separatio formae a materia. Sed separatio et unitio contraria sunt. Igitur si ex separatione destruitur res, profecto in suo esse nonnisi unitione conseruatur. 

Vnitio autem non fit nisi unitate. Quae cum ab unito separatur, unitio, qua unum erat, dissoluitur. Soluta autem unitione, destruitur essentia eius, quod ex earum unitione prouenerat, quia fit non unum. Quapropter, sicut unitate res ad esse ducitur, sic et unitate in illo esse custoditur. Vnde esse et unum inseparabiliter concomitantur se et uidentur esse simul natura.

Quia enim creator uere unus est, ideo rebus, quas condidit, hoc in munere dedit ut unaquaeque habeat esse una. Ac per hoc, quia ex quo res habeat esse una est, ideo motus omnium substantiarum est ad unum et propter unum; et nihil eorum, quae sunt, appetit esse multa, sed omnia, sicut appetunt esse, sic et unum esse. Quia enim omnia esse naturaliter appetunt, habere autem esse non possunt nisi sint unum, ideo omnia ad unum tendunt. Vnitas enim est quae unit omnia et retinet omnia diffusa in omnibus, quae sunt.

Quapropter, quia materia non habet esse nisi per unitionem sui cum forma, formam autem non tenet unitam cum materia nisi unitas, ideo materia eget unitate ad uniendum se et ad suscipiendum esse. Materia enim contraria est unitati, eo quod materia per se diffluit et de natura sua habet multiplicari, diuidi et spargi; unitas uero retinet, unit et colligit. Ac per hoc, ne materia diuidatur et spargatur, necesse est, ut ab unitate retineatur. Quidquid autem eget alio ad uniendum se, non unitur per se.

Quod autem per se non unitur, per se utique spargitur, quia omnis res, quae facit aliquam rem contrariam agentis, facit contrariam factae rei; contrariorum enim contrarii sunt effectus. Quapropter, quia unitas facit unum, profecto materia faciet diuisionem. Ac per hoc unitas per se retinet materiam. Sed quidquid per se retinet, non potest facere separationem.

Forma ergo existens in materia, quae perficit et custodit essentiam cuiusque rei, unitas est, descendens a prima unitate, quae creauit eam.

Prima enim et uera unitas, quae est unitas sibi ipsi, creauit aliam unitatem, quae esset infra eam. Sed —quia omne creatum omnino diuersum est a quo creatum est— profecto creata unitas a creante unitate omnino diuersa esse debuit et quasi opposita. Sed quia creatrix unitas non habet principium neque finem nec permutationem nec diuersitatem, ideo creatae unitati accidit multiplicitas et diuersitas et mutabilitas; ita ut in quadam materia sit habens principium et finem, in quadam uero principium et non finem, quia in quibusdam subiacet permutationi et corruptioni, in quibusdam permutationi sed non corruptioni.

In quibus enim materia est subtilis, simplex, remota a contrarietate et separatione, parificatur ei unitas et unitur cum ea sic ut haec et illa sint unum non diuisibile in actu; sicut in caelestibus corporibus, in quibus unitas a materia inseparabilis est, et ideo carent fine, quia perpertua sunt. In quibus uero materia fuerit spissa, debilis, non adaequatur ei unitas, sed debilitatur in uniendo et retinendo eorum essentiam, et ob hoc dissoluitur essentia eorum, quia non retinetur ab unitate; sicut in generatis, quae habent principium et finem. Quanto enim unaquaeque unitas fuerit propinquior primae et uerae unitati, tanto materia formata per illam erit magis una et simplicior; et e contrario, quanto remotior fuerit a prima unitate, tanto erit multiplicior et compositior.

Et ob hoc unitas, quae duxit ad esse materiam intelligentiae, est magis una et simplex, non multiplex nec diuisibilis essentialiter; sed si diuisibilis est, hoc siquidem accidentaliter est; et ideo haec unitas simplicior et magis una est omnibus unitatibus, quae ducunt ad esse ceteras substantias, eo quod immediate cohaeret primae unitati, quae creauit eam.

Sed quia unitas subsistens in materia intelligentiae est unitas io simplicitatis, ideo necessario unitas subsistens in materia animae, quia infra eam est, crescit et multiplicatur et accidit ei mutatio et diuersitas; et sic paulatim descendendo a superiore per unumquemque gradum materiae inferior unitas augetur et multiplicatur, quousque peruenitur ad materiam, quae sustinet quantitatem, scilicet substantiam huius mundi.

Quae quia a prima unitate remotissima est, ideo spissa et, corpulenta et constricta est, et propter spissitudinem et grossitudinem suam opposita est substantiae superiori, quae est subtilis et simplex, quoniam illa est subiectum principii et initii unitatis, haec uero est subiectum finis et extremitatis unitatis.

Finis uero multum distat a principio, quoniam finis non est dictus io nisi defectus uirtutis et terminus. Undea secundum descensum unitatis a superiore ad inferius fit degradatio suae simplicitatis et minoratio suae uirtutis, ad similitudinem aquae, quae in ortu suo subtilis et clara nascitur, sed paulatim deorsum defluens in paludibus et stagnis inspissatur et obscuratur. Sic paulatim uariatur unitas propter uarietatem materiae, quae sustinet eam. Nam quia aliquid materiae est spirituale et aliquid eius corporale est, aliquid eius purum et lucidum et aliquid eius est spissum et obscurum; et hoc propter quantitatem, cuius partes in aliquibus sunt rariores, ut in aere, in aliquibus uero constrictiores, ut in lapide.

Ideo unaquaeque pars materiae secundum gradum suae elongationis a prima unitatis origine recipit unitatem, qua dignior est ex sua aptitudine. Inde est, quod uidemus partes ignis nimis unitas et simplices et aequales, adeo quod forma eius uidetur una, non habens in se diuersitatem. Partes uero aeris et aquae inuenimus magis diuersas et separatas, adeo, quod partes eorum et unitates discerni possunt; in duris autem et spissis unitatis iam maior est diuersitas et obscuritas.

Quia igitur materia in supremis formata est forma intelligentiae, deinde forma rationalis animae, postea uero forma animae sensibilis, deinde inferius forma animae uegetabilis, deinde forma naturae, ad ultimum autem in infimis forma corporis, hoc non accidit ex diuersitate uirtutis agentis, sed ex aptitudine materiae suscipientis.

Formaa enim est quasi lumen, eo quod sicut per lumen res uidetur, sic per formam cognitio et scientia rei habetur, non per materiam; sed hoc lumen in quibusdam est clarius, in quibusdam uero obscurius, prout materia cui infunditur fuerit clarior uel obscurior. Quo enim materia fuerit sublimior, fit subtilior et penetratur tota a lumine; et ideo substantia ipsa fit sapientior et perfectior sicut intelligentia et rationalis anima. Et e contrario, quo materia fuerit inferior, fit spissior et obscurior et non ita tota penetratur a lumine. Quo magis enim materia descendit, sicut iam dictum est, constringitur et spissatur et corpulentatur et partes eius mediae prohibent ultimas perfecte penetrari a lumine. Non enim est possibile ut tantum luminis penetret partem secundam, quantum primam, nec ad tertiam tantum luminis peruenit, quantum ad mediam; Et sic paulatim donec perueniatur, usque ad partem infimam.

Quae, quia remotissima est a fonte luminis, lumen debilitatur in illa. Nec tamen hoc fit sicut praedictum est propter lumen in se, sed propter multam densitatem et obscuritatem materiae in se. Quemadmodum lumen solis cum admiscitur tenebroso aeri, non est illius uirtutis, cuius est admixtum claro aeri, uel quemadmodum pannus albus tenuissimus, cum induitur a corpore nigro occultatur candor eius propter abundantiam nigredinis; uel quemadmodum si tres uel plures fenestrae uitreae una post aliam recte contra radium solis disponantur in ordine, constat siquidem quod secunda minus recipit luminis quam prima, et tertia minus quam secunda, et sic usque ad ultimam fit defectus luminis non propter lumen in se, sed propter elongationem fenestrae uitreae a lumine; ita et lumen formae unitatis, quod infusum est materiae, descendendo fit debile et obscurum, ita ut primum eius multum discrepet a medio et medium ab ultimo.

Et propter hanc diuersitatem formae unitatis non uno modo sed pluribus dicitur aliquid unitate unum. Vnum enim aliud est essentiae simplicitate unum, ut Deus. Aliud simplicium coniunctione unum, ut angelus et anima quorum unumquodque est unum coniunctione materiae et formae. Aliud est continuitate unum, ut arbor uel petra. Aliud est compositione unum ut ex multis tabulis una arca uel ex multis partibus una domus. Alia dicuntur unum aggregatione, ut populus et grex, congeries lapidum uel acernus tritici. Alia dicuntur proportione unum, ut rector nauis et gubernator ciuitatis dicuntur unum, similitudine officii.

Alia dicuntur unum accidente, ut diuersa subiecta eiusdem qualitatis dicuntur unum in ea, sicut nix et cygnus unum sunt in albedine. Alia dicuntur unum numero, ut diuersa acccidentia, quae eidem subiecto insunt, dicuntur unum numero, id est in numerando, ut hoc dulce et hoc ceruleum uel hoc longum et hoc latum. Alia dicuntur unum ratione, sed hoc duobus modis, quia uel ratione consortii, ut intellectus et res et uocabulum unum genus, uel ratione unius sacramenti, ut spiritus aqua et sanguis dicuntur unum. Alia dicuntur natura unum, ut participatione speciei plures homines unus. Alia dicuntur unum natione uel lingua, ut multi homines dicuntur gens una uel una tribus. Alia dicuntur unum more, sed hoc duobus modis, quia secundum consensum uirtutis et dilectionis ut “multitudinis credentium erat cor unum et anima una”, uel  secundum consensum eiusdem uitii plures homines dicuntur unum, ut “qui adhaeret meretrici, unum corpus efficitur”.

Sic omnia unitatem appetunt, ut etiam ea quae multa sunt, unum dici uolunt. Quaecumque enim sunt id quod sunt aut uera unitate esse nituntur, aut saltem eam simulando nituntur. Quidquid enim est, uel est unum uel plura. Pluralitas autem non est nisi ex aggregatione unitatum. Quae unitates si sunt disgregatae faciunt multitudinem, si uero fuerint continuae in materia, faciunt magnitudinem. Quapropter inter unitates quantitatis discretae et unitates quantitatis continuae subsistentis in  materia nihil interest, nisi quia illae disgregatae sunt, istae uero continuae. Continuum ergo non est nisi ex disgregato, quia intellectus continuitatis in continuo non est nisi continuatio disgregatorum. Ac per hoc necesse est ut continua quantitas non adueniat in substantiam nisi ex unitatibus.

Quamcumque enim partem quantitatis signaueris necesse est ut sit unum uel plura. Sed omnis pluralitas, ut dictum est, ex unitatibus est. Unde aperte datur intelligi, quod discretae et continuae quantitatis radix una est, eo quod compositae sunt ex una re et resoluuntur ad unum; et etiam quia partes corporis, quo magis fuerint sibi coniunctae et constrictae, ipsum corpus erit spissius et magis quantum, ut lapis, et e contrario, quo magis fuerint partes corporis dissolutae et rarae, ipsum erit subtilius et leuius et minus quantum, ut aer.

Verum est igitur, quod continua quantitas non uenit in substantiam nisi ex coniunctione et constrictione unitatum in illa.

Unitas igitur est, qua unaquaeque res est una et est id quod est.