Person

Leibniz

German polymath philosopher, mathematician, and theologian (1646-1716); co-inventor of calculus (with Newton, contested); architect of the principle of sufficient reason; developer of the contingency cosmological argument in its mature form; author of the Theodicy (1710), which gave us the term "theodicy" itself.

Biography

1646, Born in Leipzig
1666, Doctor of Law, University of Altdorf (refused the doctorate at Leipzig at age 20)
1672-1676, Diplomatic career in Paris; meets the leading philosophers of the day; develops calculus independently of Newton
1676, Court librarian at Hanover (lifelong post)
1684, Publishes calculus in Acta Eruditorum
1710, Theodicy (in French, Essais de Théodicée sur la bonté de Dieu, la liberté de l'homme, et l'origine du mal)
1714, Monadology, late metaphysical synthesis
1716, Died in Hanover

Leibniz was multi-disciplinary at a level rarely matched: philosophy, mathematics (calculus, binary system, calculating machines), physics, geology, history, library science, jurisprudence, theology, linguistics, diplomacy.

Major works

Discourse on Metaphysics (1686)
New System of Nature (1695)
New Essays on Human Understanding (written 1704; published 1765), response to Locke
Theodicy (1710)
Monadology (1714), a 90-paragraph synthesis
Principles of Nature and of Grace (1714)

Major contributions

1. The Principle of Sufficient Reason (PSR)

Leibniz's foundational metaphysical principle:

"There can be found no fact that is true or existent, or any true proposition, without there being a sufficient reason for its being so and not otherwise, although we cannot know these reasons in most cases." (Monadology §32)

The PSR claims: everything that exists has a sufficient explanation for its existence, either in something else, or in the necessity of its own nature.

This is the engine of Leibniz's cosmological argument:

Every contingent thing requires explanation
The universe is contingent
The universe's explanation cannot be in itself (regress)
Therefore there must be a necessary being whose explanation is in itself, i.e., God

See Contingency Argument.

2. The contingency cosmological argument

Leibniz's contingency argument is the modern-classical statement of the cosmological argument tradition. It differs from Aquinas's earlier versions (Aquinas Five Ways) in:

Less reliance on Aristotelian act/potency metaphysics
More direct appeal to the PSR
Cleaner distinction between contingent and necessary being

This argument has been revived in modern analytic philosophy of religion by:

Richard Swinburne (The Existence of God, 1979/2004)
William Lane Craig (in cooperation with Kalam)
Alexander Pruss (The Principle of Sufficient Reason, 2006)
Joshua Rasmussen (How Reason Can Lead to God, 2019)

3. The "best of all possible worlds" theodicy

In Theodicy (1710), Leibniz answers Pierre Bayle's skeptical challenge regarding evil:

God is omniscient (knows all possible worlds)
God is omnipotent (can actualize any possible world)
God is omnibenevolent (would actualize the best)
Therefore the actual world is the best possible world

This is the most-famous version of the best of all possible worlds (BPW) thesis. Voltaire's Candide (1759) satirizes this view ruthlessly through the character Pangloss.

The Leibnizian theodicy:

Metaphysical evil (limitation, finitude) is necessary in any created world
Physical evil (suffering) is permitted because it serves greater goods
Moral evil (sin) is permitted because the freedom that produces moral good also makes moral evil possible

Leibniz's theodicy has been heavily critiqued (the actual world doesn't seem like the best possible) but remains a serious option in philosophy-of-religion debates. Modern Reformed treatments (Plantinga's free-will defense, see Problem of Evil, Free Will Defense) move away from BPW toward weaker claims (logical compatibility of God + evil), but the Leibnizian framework remains influential.

4. The monadology, substance metaphysics

Leibniz's mature metaphysics: reality is composed of monads, non-extended, mind-like, simple substances. Each monad has its own complete intrinsic perceptual life; God orchestrates pre-established harmony among them.

The monadology has minimal direct apologetic application but illustrates Leibniz's commitment to a deeply theistic metaphysics, every monad reflects the whole; God is the Monad of monads who sustains the system.

5. Modal metaphysics

Leibniz developed sophisticated modal-logic theory:

Possible worlds as collections of co-possible states of affairs
Necessary truths = true in all possible worlds
Contingent truths = true in some but not all
God's perfect knowledge of all possible worlds (foundation for Molinism)

Leibniz's modal metaphysics is foundational for:

Plantinga's modal ontological argument
Possible-worlds semantics in modern modal logic (Kripke etc.)
Molinism / middle-knowledge theology

Leibniz on the existence of God

Cosmological from contingency

The famous question (in Principles of Nature and Grace §7): "Why is there something rather than nothing?"

This question defines modern cosmological argument. Leibniz's answer: only a necessary being's existence can terminate the chain of contingent why-questions. See Contingency Argument and Aquinas Five Ways (Way 3).

Ontological argument refinement

Leibniz refined Anselm's ontological argument (New Essays IV.10): the argument works if it is possible that a most perfect being exists. Leibniz thought the possibility could be demonstrated (no contradiction in maximal compossibility of perfections). Plantinga's modern modal version develops this insight.

Leibniz in this corpus

Contingency Argument, primary modern source
Aquinas Five Ways, comparison-and-development
Modal Ontological Argument, modal-metaphysics foundation
Problem of Evil, Free Will Defense, Leibnizian-vs-Plantingan theodicy
Free Will and Determinism, Molinism foundation
Necessary Being is an Intelligent Mind

Reception history

18th-century

Christian Wolff systematized Leibniz's philosophy; "Leibniz-Wolff philosophy" dominated German universities until Kant's critique. Voltaire ridiculed BPW theodicy.

Kantian critique

Kant rejected speculative metaphysics generally; Leibniz's PSR-based cosmological argument fell under Kant's critique of speculative theology.

20th-century revival

Analytic-Christian philosophy revived Leibniz in:

Modal ontological argument (Plantinga 1974)
PSR-based cosmological argument (Pruss 2006; Rasmussen 2019)
Molinist middle-knowledge (Craig 1991)

Modern Christian philosophy treats Leibniz as a major resource alongside Aquinas.

Major secondary literature

Nicholas Jolley (ed.), The Cambridge Companion to Leibniz (1995)
Maria Rosa Antognazza, Leibniz: An Intellectual Biography (2009)
Brandon Look (ed.), The Continuum Companion to Leibniz (2011)
Alexander R. Pruss, The Principle of Sufficient Reason: A Reassessment (2006)
Robert Adams, Leibniz: Determinist, Theist, Idealist (1994)