Front Page Titles (by Subject) Deriving Ought from Is - Literature of Liberty, October/December 1978, vol. 1, No. 4
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Deriving “Ought” from “Is” - Leonard P. Liggio, Literature of Liberty, October/December 1978, vol. 1, No. 4 
Literature of Liberty: A Review of Contemporary Liberal Thought was published first by the Cato Institute (1978-1979) and later by the Institute for Humane Studies (1980-1982) under the editorial direction of Leonard P. Liggio.
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Deriving “Ought” from “Is”
“Naturalism and Thomistic Ethics.” The Thomist 40 (April 1976): 222–242.
Can we logically construct an objective human ethics that grounds moral obligations and the norms for being a “good” man in factual reality? “Prescriptivists” have joined battle with “descriptivists” (or naturalists) on this crucial issue.
Prescriptivists including R.M. Hare deny that we can legitimately derive values from such facts as human nature. Their doubts stem from Hume's fact/value dichotomy and G.E. Moore's “naturalistic fallacy”: We cannot logically derive a moral “ought” from a factual or descriptivists “is.” Seeking a single meaning for “good” behind all its different uses, prescriptivists claim that the function of “good” in moral evaluation is to emotionally commend. There is, they assert, no necessary or logical tie connecting our commending something as “good” with the descriptive qualities that we select as the criteria for an object's goodness.
On the other hand, naturalists seek to root values and the standard of “good” in the nature of things, namely man's distinctive characteristics. When we “commend” something to someone as “good” we mean to express more than our subjective emotional approval; we seek to suggest that there are sound reasons or objective qualities in the thing that should rightly commend it to someone.
Aquinas identified humans as “good” when they exercised well their distinguishing trait of the rational will to achieve specific goods because these human goods tended to perfect man's nature. Aquinas did not, as D.J. O'Connor asserts in Aquinas and Natural Law (1968), commit a fallacy moving from what men do in fact seek as ends to what they ought to seek. The Thomistic approach may circumvent Hume's stricture against derving such an “ought” from an “is.” The author formulates a way to derive human values from facts:
Thus, human good is the set of “virtues,” values, or excellences which do in fact perfect individual human nature. This set of values is hierarchically ordered; human reason and will are said to function well in defining and directing us to achieve these many and varied goods.