Stony Brook University

descriptive principle: “no one can be convinced of anything except for reasons of his own
utility; it follows that every duty must be useful.” 71 That is, these propositions fulfill the
descriptive requirement that no voluntary action is possible unless it involves awareness
of one’s own good.
These arguments, however, explain only the psychological requirement for
performing an act for the good of another person. This explains how an obligation, that
we formally have, is a possible action (i.e., that ought implies can). However, this
psychological requirement should not obscure the fact that obligation itself has its formal,
a priori, prescriptive ground in the definition of right. This has already been seen in the
implications of the beginning propositions (1, 2, and 3). But this may also be seen in the
implications of the two propositions that conclude this paragraph on Right:
We therefore derive two propositions from the common agreement of
those who use these terms: first, that everything necessary is just and,
second, that every duty (or injustice) is useful (or harmful). (LL 134) 72
Leibniz has in effect “clarified” what was implicit in the ordinary usage of the terms of
right. To recall, the common assumption at the beginning was that right involves both the
good for oneself and another. But it is important to see how these concluding
propositions depend on an implicit and unstated (at least in this paragraph) definition;
namely, the two-fold definition of right from the Nova Methodus: “right is a moral
power; obligation a moral necessity.” This definition is implied (or assumed) in the
concluding propositions in the following way: (1) “Everything necessary is just.” Why
does he speak of necessity here? He does not mean logical necessity, as if to refer to the
logical compatibility of notions; he means, rather, moral necessity. He means that we
have, as a matter of the definition of right as a moral power, a moral necessity or
obligation to the other. This has been well established in the Nova Methodus and is here
presupposed. Furthermore, not harming another is of course just, since it is the precept of
ius strictum. This proposition, i.e., (1), is understandable only on the basis of the a priori
definition of right. (2) “Every duty is useful,” also stems from the definition of right. That
we have duties to others is a logical consequence of the definition of right. We can
understand ‘duty’ (debitum) here as largely synonymous with obligatio, since these terms
are used interchangeably in this paragraph. Based on this, what then needs to be shown is
that every duty to others is consistent with one’s own utility, so that duties are
motivationally possible actions. This consistency is made possible by the descriptive
principles of motivation. Since we derive pleasure from increasing the pleasure in
another, every duty to others is useful to ourselves and motivationally possible to do. 73
71 A.6.1.461: “Ex his constat non posse qvenqvam in malum suum si rei summam ineas, obligari. Adde nec
nisi in bonum suum obligari qvenqvam. Cum enim justitia sit qviddam qvod homini prudenti persvaderi
potest, nihil autem persvaderi possit, nisi petitis rationibus ab utilitate auditoris, necesse est omne debitum
utile esse.”
72 A.6.1.461: “Habemus igitur propositiones duas: primo, omne necessarium justum est, secondo, omne
debitum (injustum) utile (damnosum) est, ex communi consensu eorum qvi vocabulis istis utuntur
derivatas.”
73 It may, however, seem odd to say that another’s pleasure is increased when I do not harm them. But if
there are duties beyond the duty not to harm (and there are, e.g., the duty to help), they will be good for
another; consequently good for us if we do them; consequently, motivationally possible for us to do.
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