“ Thus the sun which possesses light perfectly, can shine by itself; whereas the moon which has the nature of light imperfectly, sheds only a borrowed light. ”
 Thomas Aquinas, Summa Theologica (1274). copy citation

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“Now it is evident that anything that has a nature or a form or a virtue perfectly, can of itself work according to them: not, however, excluding the operation of God, Who works inwardly in every nature and in every will. On the other hand, that which has a nature, or form, or virtue imperfectly, cannot of itself work, unless it be moved by another. Thus the sun which possesses light perfectly, can shine by itself; whereas the moon which has the nature of light imperfectly, sheds only a borrowed light. Again, a physician, who knows the medical art perfectly, can work by himself; but his pupil, who is not yet fully instructed, cannot work by himself, but needs to receive instructions from him.
Accordingly, in matters subject to human reason, and directed to man's connatural end, man can work through the judgment of his reason.” source

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