Such is the assertion at the outset of T.K. Abbott's translation of Kant's The Metaphysics of Morals,
Die Metaphysik der Sitten (1797), translated by Abbott as
The Metaphysical Elements of Ethics:
If there exists on any subject a philosophy (that is, a
system of rational knowledge based on concepts), then
there must also be for this philosophy a system of pure
rational concepts, independent of any condition of in-
tuition, in other words, a metaphysic.
He goes on:
It may be asked
whether metaphysical elements are required also for
every practical philosophy, which is the doctrine of du-
ties, and therefore also for Ethics, in order to be able to
present it as a true science (systematically), not merely
as an aggregate of separate doctrines (fragmentarily).
So are metaphyics (or a metaphysical edifice) necessary for ethics? Only, perhaps, if that ethics is to be presented systematically rather than fragmentarily (a key difference between Kant and the Buddha's teachings, perhaps).
As regards pure jurisprudence, no one will question
this requirement; for it concerns only what is formal
in the elective will, which has to be limited in its exter-
nal relations according to laws of freedom; without re-
garding any end which is the matter of this will.
I'm not sure what exactly to make of this sentence. For 'pure jurisprudence', I take it to mean abstract reasoning on the good, in itself, free from particular instances. 'Without regarding any end...' I take to mean without any particular goal in mind, again trying to understand 'the good' in an abstract form. If 'the good' is to be understood on par with matter or mathematics, it must be examined in the most abstract manner possible. And so, Abbott continues:
Here, therefore, deontology is a mere scientific doctrine (doc-
trina scientiae).
Abbott goes on to make a distinction similar to Kant's own in the Groundwork, namely that our reasoning about ethics must begin with the concept of duty in its most abstract form, not from feelings, either the quest for maximizing happiness or avoiding suffering (again a distinction from Buddhism, which begins and ends with suffering). If we are to do this, we ally ourselves with 'feeling by whatever it may be excited' and thus endless possibilities in the empirical realm.
For instance a greedy drug-maker who stumbles upon a miracle drug that saves millions and accidentally releases its patent (and thus earns nothing) might be hailed as a hero as great as Jonas Salk (who did the same but purposely didn't patent the polio vaccine). The consequences of each man's work might be the same, but only one acted according to a sense of duty toward humankind. Thus, for Kant and his followers, only one acted morally.
Abbott then reiterates Kant's understanding of how morality leads to happiness, namely that a 'thinking man' conquers the temptations of vice and is conscious of having come to act according to duty, and 'finds himself in a state of peace and satisfaction which may well be called happiness, in which virtue is her own reward' (p.10). The happiness-seeker, on the other hand, argues that this happiness was the motive all along and our thinking man merely acts out of duty for the sake of that happiness.
[Now, this may look conspicuously like Buddhism, but I'd argue the opposite. The Buddhist practitioner might start with a quest to just overcome his/her own suffering, but this morphs over time with the realization of interconnectedness, etc, toward a recognition of the need to help all beings, thus abandoning a merely selfish goal and taking up a sense of duty to be of service. It is this
shift from a selfish goal toward a universal one that marks a stream-enterer; or something like that. I need to gather sources and justifications, but the point is that the Buddhist 'thinking man' who has conquered vice and done his duty and feels peace and satisfaction will have done so out of an attraction to goodness (
kusala) itself in the end.]
In any case, if the Buddhist practitioner merely seeks after whatever appears to remove suffering, he will no doubt chase after a million chimerical solutions, running this way and that. It is only with a firm grasp on certain realizations (or acceptances) about the nature of reality can he/she move forward in a straight line in life. Obeying duty (Dhamma) often means abandoning one's search for the latest happiness-producing-activity. And this is the key point. If one's allegiance is toward happiness, one will not abide by duty, which often requires suffering, temporary as it may be.
Thus the need for this metaphysical distinction between ultimate aims. As Abbott concludes:
If this distinction is not observed; if eudaemonism
(the principle of happiness) is adopted as the principle
instead of eleutheronomy (the principle of freedom of
the inner legislation), the consequence is the euthana-
sia (quiet death) of all morality. (p.11)
So we have begun with a call to the need for metaphysics and ended with that metaphysics differentiating two key directions that moral philosophers might take: one toward physiology and joys/sorrows as its justification, and one toward a transcendent, unknowable (but feelable) duty or dharma.