This defectibility supposes an equilibrium or indetermination of the will in respect to contraries which is overcome by a self-determining power. Such an equilibrium can only exist, when opposite objects, in which some good corresponding to the spontaneous tendency of the will is contained, are presented to the intellect as desirable and worthy of choice; in such a way that the motives for choice balance each other. The will must follow the intellect, and therefore an error in the choice must be preceded by an erroneous judgment, which is possible only when the object presented to it does not compel assent. Moral probation requires that there should be an obligation, arising from the eternal law of God or a positive command, to choose one of the opposite objects and reject the other. It is this which makes these objects contrary to each other in a moral respect, and is the reason why liberty of choice between them is called the liberty of contrariety, and the determination to the one is a virtuous, while that to the other is a vicious act. It is easy to understand this liberty of contrariety and the moral discipline which is requisite for its due control and direction, in respect to human nature. From its complex constitution, the sensible good is often opposed to the rational good, and reason, which ought to govern, is easily deceived by the imagination. In the case of pure spirits, it is more difficult to see how they can be subject to any illusion, or capable of undergoing any moral probation. In the natural order, they are perfect, and cannot err in the apprehension of that which is truly desirable as their chief good. They are not, therefore, capable of probation in the moral order of pure nature. But in the supernatural order, the object proposed to them being presented in an obscure, supernatural light, which does not compel assent, there is room for a suspension of the act of consent, and a power of rejecting the sovereign good by a voluntary self-determination, in adhering to the inferior object which they naturally comprehend and love. In fact, it was in this way that the fallen angels sinned and rebelled against God. In like manner, Adam, who was elevated to a perfect state like that of the angels, and enjoyed absolute dominion over all sensible concupiscence, underwent a supernatural probation, in which he fell through the seduction of Eve, who was the instrument of the demon, who had previously made her the victim of his diabolical sophistry.