Stony Brook University

As Leibniz has argued similarly in the past, the rules of justice are demonstrable “in their
full extent and perfection” only under these assumptions. 32 A number of things have been
said to follow from these assumptions (or, from the proofs, as he has also claimed): If
God exists and the soul is immortal, then we can be sure that just actions will eventually
be rewarded with happiness and unjust actions will eventually be punished. Under these
assumptions we can live under a perfectly benevolent monarch whom we would be
pleased to serve. We even ought to find pleasure in serving God, rather than in serving
ourselves. Only the pleasure we find in serving God can bring us to preserve a truly
universal moral community, rather than a community of thieves. In sum, to answer the
question of how the maxims of justice may be known through reason, the answer is this:
through demonstration. To know them through demonstration means to assume God and
immortality, and to draw conclusions that follow from these assumptions. Presumably,
these conclusions would be the maxims of justice themselves, such as “act honorably.”
Thus in this way we establish at least one sense of the claim that “moral doctrine is a
demonstrative science.” The demonstrative ground of moral doctrine lies not in the
maxims of justice themselves, but rather in the assumptions of God and immortality.
Arguably, however, there are deep difficulties with establishing moral doctrine on
these grounds. For one thing, this demonstration does not explain what a just action is. As
Leibniz has made clear in the Elementa and elsewhere, the punishability of an act does
not by itself determine the justice or injustice of an act; and Leibniz never diverges from
this view. God of course would always punish an unjust act, but the ground of an unjust
act lies in the definition of ‘just’ (or in the definition of ‘right’) and in the rules of justice
themselves. Secondly, Leibniz “demonstrates” here only this: there are strong incentives
or motives for acting justly, namely, reward and punishment. Without the motivating
assumptions of God and immortality, a community, indeed a world, would resemble a
community of self-interested and unruly thieves. Although the rules of justice are
“engraved in the soul,” either as primitive rules or as derived rules, they are not effective
unless we assume God and immortality. Leibniz uses here the same passage from Horace
that he used in 1678, when first ascribing motives to the three degrees of right. 33 The
passage suggests that as long as one can escape divine detection, one will do what is
honorable only out of self-interest. In the end, we ought to seek our pleasure in God,
because the greatest pleasure can be found in God, and in the eternal justice God will
provide. This pleasure, joy, or happiness makes us follow the rules of justice more
effectively and honorably. Therefore, “moral doctrine as a demonstrative science,” at
least at this point, is based fundamentally on motivating principles. But again, these
qu’ils en ont, plustost que sur le plaisir qu’ils y devroient prendre, qui est des plus grands lors que Dieu en
est le fondement; ceux-la sont sujets à ressembler un peu à la societé des Bandits. Sit spes fallendi
miscebunt sacra profanis.” (Fn: Horace Epîtres, I, XVI, v.54.)
32 As we saw in the Codex Juris Gentium: “Ut vero universali demonstratione conficiatur, omne honestum
esse utile, et omne turpe damnosum, assumenda est immortalitas animae, et rector universi DEUS. Ita fit ut
omnes in Civitate perfectissima vivere intelligamur, sub Monarcha, qui nec ob sapientiam falli, nec ob
potentiam vitari potest; idemque tam amabilis est, ut felicitas sit tali domino” (A.4.5.63). It is also
interesting to consider this notion of assumption in relation to Kant’s discussion in the Critique of Practical
Reason of God and immortality as theoretical “postulates,” but not demonstrable (part 1, book 2, chapter 2,
title 4).
33 See my Chapter Three on “De Justitia et Jure.”
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