Virtue is a mean between the two extremes of excess and defect, each of which is a vice. Thus an excess of wisdom becomes shrewdness and cunning and deceit; while a defect means ignorance. The true wisdom consists in the middle way between the two extremes. Similarly courage is a mean between foolhardiness and rashness on the side of excess, and cowardice on the side of defect. Temperance is a mean between excessive indulgence of the appetites on one side and utter insensibility on the other. The mean of justice is the result of the harmonious combination of the means of the last three. If the rational soul has wisdom and the two other souls are obedient to it through modesty and courage, their substance changes into the substance of the rational soul, _i. e._, their bad qualities are transformed into the four virtues just mentioned. Then the two lower souls unite with the rational soul and enjoy eternal happiness with it. On the other hand, if the rational soul follows the senses, its wisdom changes into their folly, its virtues into their vices, and it perishes with them.[145] The immortality of the soul is proved as follows. Things composed of elements return back to their elements, hence the soul also returns to its own origin. The soul is independent of the body, for its qualities, thought and knowledge, are not bodily qualities, hence they become clearer and more certain after the soul is separated from the body than before, when the body obscured its vision like a curtain. The fact that a person's mind is affected when his body is ill does not show that the soul is dependent in its nature upon the body; but that acting as it does in the body by means of corporeal organs, it cannot perform its functions properly when these organs are injured. Since death is a decree of God, it is clear that he has a purpose in changing the relations of body and soul. But if the soul comes to an end, this change would be a vain piece of work of which he cannot be guilty. Hence it follows that the destruction of the body is in order that we may exist in another similar form, similar to the angels.[146] The purpose of the soul's coming into this world is in order that she may purify the two lower souls; also that she may know the value of her own world in comparison with this one, and in grieving for having left it may observe God's commandments, and thus achieve her return to her own world. In the matter of returning to their own world after separation from the body, souls are graded according to the measure of their knowledge and the value of their conduct. These two conditions, ethical and spiritual or intellectual, are requisite of fulfilment before the soul can regain its original home. The soul on leaving this world is like a clean, white garment soaked in water. If the water is clean, it is easy to dry the garment, and it becomes even cleaner than it was before. But if the water is dirty, no amount of drying will make the garment clean.