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Aquinas' four concepts of law: defining right and wrong
In Aquinas' natural law theory goodness is intrinsic to the act, as defined by purpose and by the eternal law of God. In an ideal world all four ideas of law would harmonise: God's purpose, and our human laws, for example, would agree. For a clear discussion of the four concepts of law by Professor Richard Jacobs go to:
Aquinas is arguing that the end or purpose of an action defines whether it is right or wrong, and that this end or purpose is revealed by the divine law and confirmed by the natural law.
So every will acting contrary to reason (even if the reason is in error) is evil, and some acts of will which stem from reason may be evil if the will is in error, so an act can be wrong if:
Wrong actions therefore break the law, and law to Aquinas has four meanings:
Right actions should conform to all four meanings of law, but ultimately it is our reason which confimrs again whether law is "just" and "right".
An Analogy might help
Imagine you receive a new BMW mini for your birthday. You find that the owner's manual is missing,so you decide to ask a mechanic friend, who's wise with cars, to come round and sort out for you what everything does. What does he do? He looks under the bonnet, tries the various switches, tests the brakes and then shows you how the car works. By observation he has worked out what's would be in the manual. The more skilled he is as a mechanic the better his observations will be.
The Eternal Law is the car manual (existing in God's mind).
The Divine Law is like an old, incomplete manual (The Bible has gaps and errors in it).
The Natural Law is what the mechanic both observes and knows innately (so we work out what the instructions are, and the more skilled we are, the better the manual from observation will be. But the natural law in the synderesis rule is also innate).