Did St. Thomas Aquinas Justify the Transition from 'Is' to 'Ought'

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0. A Short History of the Problem
In every system of morality, which I have hitherto met with, I have always remark’d,
that the author proceeds for some time in the ordinary way of reasoning, and
establishes the being of a God, or makes observations concerning human affairs;
when of a sudden I am surpriz’d to find, that instead of the usual copulations of
propositions, is, and is not, I meet with no proposition that is not connected with an
ought, or an ought not. This change is imperceptible; but is, however, of the last
consequence. For as this ought, or ought not, expresses some new relation or
affirmation, it is necessary that it should be observ’d and explain’d; and at the same
time that a reason should be given, for what seems altogether inconceivable, how this
new relation can be a deduction from others, which are entirely different from it. But
as authors do not commonly use this precaution, I shall presume to recommend it to
the readers; and am persuaded, that this small attention wou’d subvert all the vulgar
systems of morality, and let us see, that the distinction of vice and virtue is not
founded merely on the relations of objects, nor is perceiv’d by reason (T, 302).
This argument caused a furore in philosophy and is known to this day as “Hume’s
Law.” It is still regarded as controversial and is usually understood as follows: it is
impossible to infer an “ought” directly from an “is.” This is to say, that between “ought”
and “is” there is so considerable a dichotomy that it is impossible to deduce imperative
phrases from descriptive ones. There is thus an inferential gap between moral and
nonmoral claims (the “Is/Ought Gap”) or between fact and value. 24 No moral
conclusion can be derived from exclusively nonmoral premises. Hence, a moral
conclusion can be derived only from premises containing at least one moral statement.
Although Hume himself did not do so, we may specify the following on the basis of his
writings: the inferential gap is accompanied by a logically independent belief about the
inherently motivational character of moral considerations. This latter belief is called an
“internalist assumption.” The “internalist assumption” is based on a commonsense view
that “normal” people will not be indifferent to moral considerations. Within Hume’s
system, this assumption is developed into the suggestion that we should derive moral
rules from the “moral sense.” Moral sense for Hume is the natural capacity to feel a
distinctive kind of approbation and disapprobation when considering features of
character from “the general point of view” (that is, independently of one’s own self-
interest). This feeling is activated primarily by natural sympathy with those who are
affected by the character traits in question. He argues, therefore, that values are the
projections of natural human desires or sentiments. Moral values are one kind of value
and their main characteristic is that they are the projections of desires that aim at the
common good of society. Through the observation and analysis of these desires or
24 For an example of the diverse interpretations of this thesis see W. D. Hudson (ed.), The Is/Ought
Thesis, (New York: Macmillan, 1969).