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

1. Aquinas’s General Approach to Cognition
The claim that St. Thomas did not infer moral rules from metaphysics or natural
science seems to be motivated by a desire to exculpate him from the charge of being an
essentialist or deductivist. Indeed, in his most mature work, the Summa theologiae
(further quoted as STh), one may be struck easily by the fact that everything seems to
follow from the established truth of God’s existence and other truths established in the
first questions of this grand work. When read in this spirit, an impression may impose
itself on the reader: that of an arbitrary juggling with abstract ideas that have no
reference to reality. Moreover, this juggling may appear to be trickery, aimed to conceal
the fact that arguments are proposed in order to increase the plausibility of accepted
dogmatic claims.
This is perhaps why some interpretations emphasize that the second part of
STh might be detached from the first without detriment to the whole of St. Thomas’s
moral project. Some authors are content to find, in this second part of STh, some first
principles that have, at last, a relationship to human experience and hence, it is thought,
one can begin to build an ethics which is both autonomous and plausible. This may
especially appear to be the case because Aquinas refers to first, indemonstrable
principles. If they are indemonstrable, no previous science is needed. This is in
accordance with the merciless alternative that ethics should be autonomous or it will not
be ethics at all.
It is apposite, in light of the above, that our presentation of Aquinas’s logic be
preceded with some general remarks about his approach to cognition and about his way