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Journal of Philosophical Research

Volume 17, 1992

James F. Fieser
Pages 155-172

The Logic of Natural Law in Aquinas’s “Treatise on Law”

Against recent commentators such as Annstrong, D’Arcy, Copleston, O’Connor, Bourke, and Grisez, I argue that the logic referred to by Thomas in his “Treatise on Law” should not be understood metaphorically. Instead, it involves a chain of syllogisms, beginning with the synderesis principle, followed by primary, secondary, and tertiary principles, and ends with a practical syllogism. In showing this, I attack the view that the synderesis principle, “good ought to be done and evil avoided,” is tautological . Second, I show the syllogistic relation between this and the more subordinate moral principIes. Finally, I argue that the practical syllogism also involves a logical deduction, where the minor premise is a propositional attitude of perception, and the conclusion is an action which expresses a proposition. What emerges is a more precise account of how actions are related to natural law.

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