1 Objection 1: It would seem that the angel's act of understanding is his substance. For the angel is both higher and simpler than the active intellect of a soul. But the substance of the active intellect is its own action; as is evident from Aristotle De Anima iii) and from his Commentator. Therefore much more is the angel's substance his action, that is, his act of understanding.
1 Ad primum sic proceditur. Videtur quod intelligere Angeli sit eius substantia. Angelus enim est sublimior et simplicior quam intellectus agens animae. Sed substantia intellectus agentis est sua actio; ut patet in III de anima per Aristotelem, et eius Commentatorem. Ergo, multo fortius, substantia Angeli est sua actio, quae est intelligere.
2 Objection 2: Further, the Philosopher says Metaph. xii, text 39) that the action of the intellect is life. But since in living things to live is to be, as he says De Anima ii, text 37), it seems that life is essence. Therefore the action of the intellect is the essence of an angel who understands.
2 Praeterea, philosophus dicit, in XII Metaphys., quod actio intellectus est vita. Sed cum vivere sit esse viventibus, ut dicitur in II de anima, videtur quod vita sit essentia. Ergo actio intellectus est essentia intelligentis Angeli.
3 Objection 3: Further, if the extremes be one, then the middle does not differ from them; because extreme is farther from extreme than the middle is. But in an angel the intellect and the object understood are the same, at least in so far as he understands his own essence. Therefore the act of understanding, which is between the intellect and the thing understood, is one with the substance of the angel who understands.
3 Praeterea, si extrema sunt unum, medium non differt ab eis, quia extremum magis distat ab extremo, quam medium. Sed in Angelo idem est intellectus et intellectum, ad minus inquantum intelligit essentiam suam. Ergo intelligere, quod cadit medium inter intellectum et rem intellectam, est idem cum substantia Angeli intelligentis.
4 On the contrary, The action of anything differs more from its substance than does its existence. But no creature's existence is its substance, for this belongs to God only, as is evident from what was said above Question 3, Article 4). Therefore neither the action of an angel, nor of any other creature, is its substance.
4 Sed contra, plus differt actio rei a substantia eius, quam ipsum esse eius. Sed nullius creati suum esse est sua substantia, hoc enim solius Dei proprium est, ut ex superioribus patet. Ergo neque Angeli, neque alterius creaturae, sua actio est eius substantia.
5 I answer that, It is impossible for the action of an angel, or of any creature, to be its own substance. For an action is properly the actuality of a power; just as existence is the actuality of a substance or of an essence. Now it is impossible for anything which is not a pure act, but which has some admixture of potentiality, to be its own actuality: because actuality is opposed to potentiality. But God alone is pure act. Hence only in God is His substance the same as His existence and His action. Besides, if an angel's act of understanding were his substance, it would be necessary for it to be subsisting. Now a subsisting act of intelligence can be but one; just as an abstract thing that subsists. Consequently an angel's substance would neither be distinguished from God's substance, which is His very act of understanding subsisting in itself, nor from the substance of another angel. Also, if the angel were his own act of understanding, there could then be no degrees of understanding more or less perfectly; for this comes about through the diverse participation of the act of understanding.
5 Respondeo dicendum quod impossibile est quod actio Angeli, vel cuiuscumque alterius creaturae, sit eius substantia. Actio enim est proprie actualitas virtutis; sicut esse est actualitas substantiae vel essentiae. Impossibile est autem quod aliquid quod non est purus actus, sed aliquid habet de potentia admixtum, sit sua actualitas, quia actualitas potentialitati repugnat. Solus autem Deus est actus purus. Unde in solo Deo sua substantia est suum esse et suum agere. Praeterea, si intelligere Angeli esset sua substantia, oporteret quod intelligere Angeli esset subsistens. Intelligere autem subsistens non potest esse nisi unum; sicut nec aliquod abstractum subsistens. Unde unius Angeli substantia non distingueretur neque a substantia Dei, quae est ipsum intelligere subsistens; neque a substantia alterius Angeli. Si etiam Angelus ipse esset suum intelligere, non possent esse gradus in intelligendo perfectius et minus perfecte, cum hoc contingat propter diversam participationem ipsius intelligere.
6 Reply to Objection 1: When the active intellect is said to be its own action, such predication is not essential, but concomitant, because, since its very nature consists in act, instantly, so far as lies in itself, action accompanies it: which cannot be said of the passive intellect, for this has no actions until after it has been reduced to act.
6 Ad primum ergo dicendum quod, cum dicitur quod intellectus agens est sua actio, est praedicatio non per essentiam, sed per concomitantiam, quia cum sit in actu eius substantia, statim quantum est in se, concomitatur ipsam actio. Quod non est de intellectu possibili, qui non habet actiones nisi postquam fuerit factus in actu.
7 Reply to Objection 2: The relation between life and to live is not the same as that between essence and to be; but rather as that between a race and to run, one of which signifies the act in the abstract, and the other in the concrete. Hence it does not follow, if to live is to be, that life is essence. Although life is sometimes put for the essence, as Augustine says De Trin. x), Memory and understanding and will are one essence, one life: yet it is not taken in this sense by the Philosopher, when he says that the act of the intellect is life.
7 Ad secundum dicendum quod vita non hoc modo se habet ad vivere, sicut essentia ad esse; sed sicut cursus ad currere, quorum unum significat actum in abstracto, aliud in concreto. Unde non sequitur si vivere sit esse, quod vita sit essentia. Quamvis etiam quandoque vita pro essentia ponatur; secundum quod Augustinus dicit, in libro de Trin., quod memoria et intelligentia et voluntas sunt una essentia, una vita. Sed sic non accipitur a philosopho cum dicit quod actio intellectus est vita.
8 Reply to Objection 3: The action which is transient, passing to some extrinsic object, is really a medium between the agent and the subject receiving the action. The action which remains within the agent, is not really a medium between the agent and the object, but only according to the manner of expression; for it really follows the union of the object with the agent. For the act of understanding is brought about by the union of the object understood with the one who understands it, as an effect which differs from both.
8 Ad tertium dicendum quod actio quae transit in aliquid extrinsecum, est realiter media inter agens et subiectum recipiens actionem. Sed actio quae manet in agente, non est realiter medium inter agens et obiectum, sed secundum modum significandi tantum, realiter vero consequitur unionem obiecti cum agente. Ex hoc enim quod intellectum fit unum cum intelligente, consequitur intelligere, quasi quidam effectus differens ab utroque.