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

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0. A Short History of the Problem
we learn that demonstrative reasoning alone is never the cause of any action. It can,
however, affect action, but only by helping to formulate the process of reasoning
scientifically (in his mind this means mathematically) and by helping to judge causes
and effects. Nor can probable reasoning motivate action, because if there were no
objects affecting our emotion of aversion or propensity (these emotions arise from the
prospect of pain and pleasure), reasoning would affect no action. Probable reasoning, to
which it is proper to discover the relation of cause and effect, can nonetheless serve to
direct action by showing the means to a desired end. Yet we are reminded that “the
impulse arises not from reason.” If the prospect of pain and pleasure alone is capable of
motivating our actions, then in the presence of an object which is indifferent to us, no
discovery of the cause-effect connection is able to motivate us. It follows for Hume that
“reason alone can never produce any action, or give rise to volition” (T, 266).
From this conclusion Hume infers also that reason is “incapable of preventing
volition, or of disputing the preference with any passion or emotion” (ibid.). We are
informed that “this consequence is necessary.” Why? Because “nothing can oppose or
retard the impulse of passion, but a contrary impulse.” It was established that reason has
no original influence; therefore, it cannot exert an opposite influence either. This
suffices for Hume to conclude that “we speak not strictly and philosophically when we
talk of the combat of passion and of reason. Reason is, and ought only to be the slave of
the passions, and can never pretend to any other office than to serve and obey them”
(ibid.). 19 Only when a false judgment accompanies a passion, can we speak about
unreasonable passions, yet even then it is not passion but judgment that is unreasonable.
The consequence is simple: “it is impossible, that reason and passion can ever oppose
each other” (T, 267).
To this it should be added that for Hume reason alone cannot even discern
between moral good and evil (cf. T, 294). 20 It follows for Hume from the simple fact
that morals do influence human actions whereas, as established, reason alone “can never
have any such influence.” Since “reason of itself is utterly impotent” in exciting
passions or actions, it is impossible that the rules of morality be its conclusions. He
believes that reason is inactive in itself and “it must remain so in all its shapes and
appearences, whether it exerts itself in natural or moral subjects, whether it considers
19 Cf. Garrett, SECT10. Note how smoothly “is” is transformed into “ought” despite the prohibition.
20 For a more ample account see e.g. Garrett, SECT11.