shed by a
single act of realization, that the world has need of conservation, _i.e._,
of continuous creation.]

This law is the special expression of a more general one, the principle
of sufficient reason, which Leibnitz added, as of equal authority, to the
Aristotelian laws of thought. Things or events are real (and assertions
true) when there is a sufficient reason for their existence, and for their
determinate existence. The _principium rationis sufficientis_ governs our
empirical knowledge of contingent truths or truths of fact, while, on
the other hand, the pure rational knowledge of necessary or eternal
(mathematical and metaphysical) truths rests on the _principium
contradictionis_. The principle of contradiction asserts, that is, whatever
contains a contradiction is false or impossible; whatever contains no
contradiction is possible; that whose opposite contains a contradiction
is necessary. Or positively formulated as the principle of identity,
everything and every representative content is identical with itself.[2]
Upon this antithesis between the rational laws of contradiction and
sufficient reason--which, however, is such only for us men, while the
divine spirit, which cognizes all things _a priori_, is able to reduce even
the truths of fact to the eternal truths--Leibnitz bases his distinction
between two kinds of necessity. That is metaphysically necessary whose
opposite involves a contradiction; that is morally necessary or contingent
which, on account of its fitness, is preferred by God to its (equally
conceivable) opposite. To the latter class belongs, further, the physically
necessary: the necessity of the laws of nature is only a conditional
necessity (conditioned by the choice of the best); they are contingent
truths or truths of fact. The principle of sufficient reason holds for
efficient as well as for final causes, and between the two realms there is,
according to Leibnitz, the most complete correspondence. In the material
world every particular must be explained in a purely mechanical way, but
the totality of the laws of nature, the universal mechanism itself, cannot
in turn be mechanically explained, but only on the basis of finality, so
that the mechanical point of view is comprehended in, and subordinated
to, the teleological. Thus it becomes clear how Leibnitz in the _ratio
sufficiens_ has final causes chiefly in mind.

[Footnote 2: Within the knowledge of reason, as well as in experiential