In the 18^th chapter, Kuyper analyzes the nature of Adam’s original righteousness. He first considers the Roman Catholic position: In Adam’s pre-Fall state, he consisted of body and spirit, horse and rider. The body, the horse, was possessed by original nature of concupiscence: desire was in man by God’s creation:

This tendency, called concupiscence, was not itself sin, but could easily become the occasion and fuel for sin. (But cf. Rom. 7:8; Col. 3:5; 1 Thess. 4:5, Auth. Ver.). Man, then, as he was originally constituted, was by nature without positive holiness, but also without sin, though burdened with a tendency which might easily result in sin.

L. Berkhof, Systematic Theology (Grand Rapids, MI: Wm. B. Eerdmans publishing co., 1938), 209. Thus, to keep man in order God gave the spirit as a rider for the horse. But since this would easily let men fail, God also bestowed a supernatural grace upon man of righteousness. As Charles Hodge explains:

According to their theory, God created man soul and body. These two constituents of his nature are naturally in conflict. To preserve the harmony between them, and the due subjection of the flesh to the spirit, God gave man the supernatural gift of original righteousness. It was this gift that man lost by his fall; so that since the apostasy he is in the state in which Adam was before he was invested with this supernatural endowment. 

Charles Hodge, Systematic Theology, vol. 2 (Oak Harbor, WA: Logos Research Systems, Inc., 1997), 103. Thus, freedom of choice remained in humanity after the Fall:

But the capacity to choose by one’s free will nevertheless continued in the sinful part of the human spirit, and today free will remains the starting point of moving toward spiritual perfection, if not in the Pelagian sense then at least in the manner of the semi-Pelagians.

Thus, the conflict within the human being is a conflict between desire and reason; and it was by the addition of a supernatural grace that Adam was in a state of original righteousness. 

The Reformed view differs at this point. Original righteousness was part of the original nature of humanity; it was not added by supernatural power. The fall of the Fall was not the loss of supernatural grace but rather corruption. He cites to Lord’s Day question 7 of the Heidelberg Catechism, “hence our nature is become so corrupt.” 

Kuyper insists that it was not the loss of essence but the corruption of nature, the two terms being distinguished:

Essence and nature, so they maintained, must be distinguished. The essence is the abiding, while nature is the changeable, such that sin did change the functioning of the nature of man, but the essence of man has remained what it was, and will remain so, even if it descends forever into the place of damnation. In Satan as well, the essence of the angel remains unchangeably the same; only his nature has, with regard to its function, changed completely into its opposite. The same is equally true of mankind.

As he works through the warrants for these positions, Kuyper first notes that the Roman view implies that man was defective in that he needed an additional to be holy. The second argument is that man in and of himself was defective in this respect then some of a different kind must be added to keep him in line; as if an angel were given to protect him. 

There is an interesting implication of this distinction: is a desire toward something in and of itself. In Roman Catholicism a desire without acquiescence of the will is not sinful – because the capacity for such desire is inherent in the human being. As the Roman Catholic Encyclopedia explains:

From the explanation given, it is plain that the opposition between appetite and reason is natural in man, and that, though it be an imperfection, it is not a corruption of human nature. Nor have the inordinate desires (actual concupiscence) or the proneness to them (habitual concupiscence) the nature of sin; for sin, being the free and deliberate transgression of the law of God, can be only in the rational will; though it be true that they are temptations to sin, becoming the stronger and the more frequent the oftener they have been indulged.  

Bavinck explains the development of this position in Scholatisticism:

Scholasticism, furthermore, began gradually to distinguish between primo-primi, secundo-primi, and plane deliberati desires, that is, those thoughts and desires that arise in us spontaneously before any consent of the will and are not at all sinful; those against which the will has offered resistance but by which it has been overpowered and which are venial sins; and those to which the will has consciously and fully consented and which are mortal sins. Added to this was the fact that the conception of original sin was becoming ever weaker and original sin itself viewed as wholly eradicated by baptism. What remained, concupiscence, was itself not sinful but only a “possible incentive to sin.” Rome, accordingly, decreed that the guilt and pollution of original sin was totally removed by baptism, that though concupiscence remained, it does not injure those who do not consent to it and can only be called sin “because it is of sin, and inclines to sin.”

Herman Bavinck, John Bolt, and John Vriend, Reformed Dogmatics: Sin and Salvation in Christ, vol. 3 (Grand Rapids, MI: Baker Academic, 2006), 142–143. But in Reformed doctrine, the desire itself is sinful; such sinful desires are culpable before God:

The idea that original righteousness was supernaturally added to man’s natural constitution, and that its loss did not detract from human nature, is an un-Scriptural idea, as was pointed out in our discussion of the image of God in man. According to the Bible concupiscence is sin, real sin, and the root of many sinful actions. 

L. Berkhof, Systematic Theology (Grand Rapids, MI: Wm. B. Eerdmans publishing co., 1938), 236. This had an interesting playout in the Reformed world with the Revoice Conference. You can read about it here.