Thomas Aquinas, Summa Theologiae

Objection 1: It seems that God is not the same as His essence or nature. For nothing can be in itself. But the substance or nature of God---i.e. the Godhead---is said to be in God. Therefore it seems that God is not the same as His essence or nature.

Objection 2: Further, the effect is assimilated to its cause; for every agent produces its like. But in created things the "suppositum" is not identical with its nature; for a man is not the same as his humanity. Therefore God is not the same as His Godhead.

On the contrary, It is said of God that He is life itself, and not only that He is a living thing: "I am the way, the truth, and the life" (Jn. 14:6). Now the relation between Godhead and God is the same as the relation between life and a living thing. Therefore God is His very Godhead.

I answer that, God is the same as His essence or nature. To understand this, it must be noted that in things composed of matter and form, the nature or essence must differ from the "suppositum," because the essence or nature connotes only what is included in the definition of the species; as, humanity connotes all that is included in the definition of man, for it is by this that man is man, and it is this that humanity signifies, that, namely, whereby man is man. Now individual matter, with all the individualizing accidents, is not included in the definition of the species. For this particular flesh, these bones, this blackness or whiteness, etc., are not included in the definition of a man. Therefore this flesh, these bones, and the accidental qualities distinguishing this particular matter, are not included in humanity; and yet they are included in the thing which is man. Hence the thing which is a man has something more in it than has humanity. Consequently humanity and a man are not wholly identical; but humanity is taken to mean the formal part of a man, because the principles whereby a thing is defined are regarded as the formal constituent in regard to the individualizing matter. On the other hand, in things not composed of matter and form, in which individualization is not due to individual matter---that is to say, to "this" matter---the very forms being individualized of themselves---it is necessary the forms themselves should be subsisting "supposita." Therefore "suppositum" and nature in them are identified. Since God then is not composed of matter and form, He must be His own Godhead, His own Life, and whatever else is thus predicated of Him.

Reply to Objection 1: We can speak of simple things only as though they were like the composite things from which we derive our knowledge. Therefore in speaking of God, we use concrete nouns to signify His subsistence, because with us only those things subsist which are composite; and we use abstract nouns to signify His simplicity. In saying therefore that Godhead, or life, or the like are in God, we indicate the composite way in which our intellect understands, but not that there is any composition in God.

Reply to Objection 2: The effects of God do not imitate Him perfectly, but only as far as they are able; and the imitation is here defective, precisely because what is simple and one, can only be represented by divers things; consequently, composition is accidental to them, and therefore, in them "suppositum" is not the same as nature.

Articulus 3

Argumentum 1

Ad tertium sic proceditur. Videtur quod non sit idem deus quod sua essentia vel natura. Nihil enim est in seipso. Sed essentia vel natura dei, quae est deitas, dicitur esse in deo. Ergo videtur quod deus non sit idem quod sua essentia vel natura.

Argumentum 2

Praeterea, effectus assimilatur suae causae, quia omne agens agit sibi simile. Sed in rebus creatis non est idem suppositum quod sua natura, non enim idem est homo quod sua humanitas.

Ergo nec deus est idem quod sua deitas.

Sed Contra

Contra, de deo dicitur quod est vita, et non solum quod est vivens, ut patet Ioan. XIV, ego sum via, veritas et vita.

Sicut autem se habet vita ad viventem, ita deitas ad deum.

Ergo deus est ipsa deitas.

Corpus

Respondeo dicendum quod deus est idem quod sua essentia vel natura. Ad cuius intellectum sciendum est, quod in rebus compositis ex materia et forma, necesse est quod differant natura vel essentia et suppositum. Quia essentia vel natura comprehendit in se illa tantum quae cadunt in definitione speciei, sicut humanitas comprehendit in se ea quae cadunt in definitione hominis, his enim homo est homo, et hoc significat humanitas, hoc scilicet quo homo est homo. Sed materia individualis, cum accidentibus omnibus individuantibus ipsam, non cadit in definitione speciei, non enim cadunt in definitione hominis hae carnes et haec ossa, aut albedo vel nigredo, vel aliquid huiusmodi. Unde hae carnes et haec ossa, et accidentia designantia hanc materiam, non concluduntur in humanitate. Et tamen in eo quod est homo, includuntur, unde id quod est homo, habet in se aliquid quod non habet humanitas. Et propter hoc non est totaliter idem homo et humanitas, sed humanitas significatur ut pars formalis hominis; quia principia definientia habent se formaliter, respectu materiae individuantis. In his igitur quae non sunt composita ex materia et forma, in quibus individuatio non est per materiam individualem, idest per hanc materiam, sed ipsae formae per se individuantur, oportet quod ipsae formae sint supposita subsistentia. Unde in eis non differt suppositum et natura. Et sic, cum deus non sit compositus ex materia et forma, ut ostensum est, oportet quod deus sit sua deitas, sua vita, et quidquid aliud sic de deo praedicatur.

Ad 1

Ad primum ergo dicendum quod de rebus simplicibus loqui non possumus, nisi per modum compositorum, a quibus cognitionem accipimus. Et ideo, de deo loquentes, utimur nominibus concretis, ut significemus eius subsistentiam, quia apud nos non subsistunt nisi composita, et utimur nominibus abstractis, ut significemus eius simplicitatem. Quod ergo dicitur deitas vel vita, vel aliquid huiusmodi, esse in deo, referendum est ad diversitatem quae est in acceptione intellectus nostri; et non ad aliquam diversitatem rei.

Ad 2

Ad secundum dicendum quod effectus dei imitantur ipsum, non perfecte, sed secundum quod possunt. Et hoc ad defectum imitationis pertinet, quod id quod est simplex et unum, non potest repraesentari nisi per multa, et sic accidit in eis compositio, ex qua provenit quod in eis non est idem suppositum quod natura.