Concerning Man
In this way, then, God brought into existence mental essence, by which I mean, angels and all the heavenly orders. For these clearly have a mental and incorporeal nature: “incorporeal” I mean in comparison with the denseness of matter. For the Deity alone in reality is immaterial and incorporeal. But further He created in the same way sensible essence, that is heaven and earth and the intermediate region; and so He created both the kind of being that is of His own nature (for the nature that has to do with reason is related to God, and apprehensible by mind alone), and the kind which, inasmuch as it clearly falls under the province of the senses, is separated from Him by the greatest interval. And it was also fit that there should be a mixture of both kinds of being, as a token of still greater wisdom and of the opulence of the Divine expenditure as regards natures, as Gregorius, the expounder of God's being and ways, puts it, and to be a sort of connecting link between the visible and invisible natures. And by the word “fit” I mean, simply that it was an evidence of the Creator's will, for that will is the law and ordinance most meet, and no one will say to his Maker, “Why have You so fashioned me?” For the potter is able at his will to make vessels of various patterns out of his clay, as a proof of his own wisdom.
Now this being the case, He creates with His own hands man of a visible nature and an invisible, after His own image and likeness: on the one hand man's body He formed of earth, and on the other his reasoning and thinking soul He bestowed upon him by His own inbreathing, and this is what we mean by “after His image.” For the phrase “after His image” clearly refers to the side of his nature which consists of mind and free will, whereas “after His likeness” means likeness in virtue so far as that is possible.
Further, body and soul were formed at one and the same time, not first the one and then the other, as Origen so senselessly supposes.
God then made man without evil, upright, virtuous, free from pain and care, glorified with every virtue, adorned with all that is good, like a sort of second microcosm within the great world, another angel capable of worship, compound, surveying the visible creation and initiated into the mysteries of the realm of thought, king over the things of earth, but subject to a higher king, of the earth and of the heaven, temporal and eternal, belonging to the realm of sight and to the realm of thought, midway between greatness and lowliness, spirit and flesh: for he is spirit by grace, but flesh by overweening pride: spirit that he may abide and glorify his Benefactor, and flesh that he may suffer, and suffering may be admonished and disciplined when he prides himself in his greatness: here, that is, in the present life, his life is ordered as an animal's, but elsewhere, that is, in the age to come, he is changed and— to complete the mystery— becomes deified by merely inclining himself towards God; becoming deified, in the way of participating in the divine glory and not in that of a change into the divine being.
But God made him by nature sinless, and endowed him with free will. By sinless, I mean not that sin could find no place in him (for that is the case with Deity alone), but that sin is the result of the free volition he enjoys rather than an integral part of his nature; that is to say, he has the power to continue and go forward in the path of goodness, by co-operating with the divine grace, and likewise to turn from good and take to wickedness, for God has conceded this by conferring freedom of will upon him. For there is no virtue in what is the result of mere force.
The soul, accordingly, is a living essence, simple, incorporeal, invisible in its proper nature to bodily eyes, immortal, reasoning and intelligent, formless, making use of an organised body, and being the source of its powers of life, and growth, and sensation, and generation, mind being but its purest part and not in any wise alien to it; (for as the eye to the body, so is the mind to the soul); further it enjoys freedom and volition and energy, and is mutable, that is, it is given to change, because it is created. All these qualities according to nature it has received of the grace of the Creator, of which grace it has received both its being and this particular kind of nature.
Marg. The different applications of “incorporeal.” We understand two kinds of what is incorporeal and invisible and formless: the one is such in essence, the other by free gift: and likewise the one is such in nature, and the other only in comparison with the denseness of matter. God then is incorporeal by nature, but the angels and demons and souls are said to be so by free gift, and in comparison with the denseness of matter.
Further, body is that which has three dimensions, that is to say, it has length and breadth and depth, or thickness. And every body is composed of the four elements; the bodies of living creatures, moreover, are composed of the four humours.
Now there are, it should be known, four elements: earth which is dry and cold: water which is cold and wet: air which is wet and warm: fire which is warm and dry. In like manner there are also four humours, analogous to the four elements: black bile, which bears an analogy to earth, for it is dry and cold: phlegm, analogous to water, for it is cold and wet: blood, analogous to air, for it is wet and warm: yellow bile, the analogue to fire, for it is warm and dry. Now, fruits are composed of the elements, and the humours are composed of the fruits, and the bodies of living creatures consist of the humours and dissolve back into them. For every thing that is compound dissolves back into its elements.
Marg. That man has community alike with inanimate things and animate creatures, whe ther they are devoid of or possess the faculty of reason.
Man, it is to be noted, has community with things inanimate, and participates in the life of unreasoning creatures, and shares in the mental processes of those endowed with reason. For the bond of union between man and inanimate things is the body and its composition out of the four elements: and the bond between man and plants consists, in addition to these things, of their powers of nourishment and growth and seeding, that is, generation: and finally, over and above these links man is connected with unreasoning animals by appetite, that is anger and desire, and sense and impulsive movement.
There are then five senses, sight, hearing, smell, taste, touch. Further, impulsive movement consists in change from place to place, and in the movements of the body as a whole and in the emission of voice and the drawing of breath. For we have it in our power to perform or refrain from performing these actions.
Lastly, man's reason unites him to incorporeal and intelligent natures, for he applies his reason and mind and judgment to everything, and pursues after virtues, and eagerly follows after piety, which is the crown of the virtues. And so man is a microcosm.
Moreover, it should be known that division and flux and change are peculiar to the body alone. By change, I mean change in quality, that is in heat and cold and so forth: by flux, I mean change in the way of depletion, for dry things and wet things and spirit suffer depletion, and require repletion: so that hunger and thirst are natural affections. Again, division is the separation of the humours, one from another, and the partition into form and matter.
But piety and thought are the peculiar properties of the soul. And the virtues are common to soul and body, although they are referred to the soul as if the soul were making use of the body.
Source: Exposition of the Faith (New Advent)