The principle of sufficient reason states that anything that happens does so for a definite reason. It is usually attributed to Gottfried Leibniz. In other words, it denies that contingent events are really so, rather than a description of our ignorance of their detailed causes. It is therefore strongly linked to ideas of determinism. Its importance is mainly historical, in relation to the debate on determinism, and Continental rationalism in general.

"Thus the sufficient reason, which needs no other reason, must be outside this series of contingent things, and must be found in a substance which is its cause, and which is a necessary being, carrying the reason of its existence with itself. Otherwise, we would not yet have a sufficient reason where one could end the series."

In fact Leibniz opposed fatalism and had a more nuanced and characteristic version of the principle, in which the contingent was admitted on the basis of infinitary reasons, to which God had access but humans did not. He explained this while discussing the problem of the future contingents:

We have said that the concept of an individual substance Leibniz also uses the term haecceity includes once for all everything which can ever happen to it and that in considering this concept one will be able to see everything which can truly be said concerning the individual, just as we are able to see in the nature of a circle all the properties which can be derived from it. But does it not seem that in this way the difference between contingent and necessary truths will be destroyed, that there will be no place for human liberty, and that an absolute fatality will rule as well over all our actions as over all the rest of the events of the world? To this I reply that a distinction must be made between that which is certain and that which is necessary. (§13, Discourse on Metaphysics)

Without this qualification, the principle can be seen as a description of a certain notion of closed system, in which there is no 'outside' to provide unexplained events with causes. It is also in tension with the paradox of Buridan's ass.

The principle was one of the four recognised laws of thought, that held a place in European pedagogy of logic and reasoning (and, to some extent, philosophy in general) in the eighteenth and nineteenth century. It was influential in the thinking of Leo Tolstoy, amongst others, in the elevated form that history could not be accepted as random.

Schopenhauer's Four Forms

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According to Schopenhauer's On the Fourfold Root of the Principle of Sufficient Reason, there are four distinct forms of the principle.

• Principle of Sufficient Reason of Becoming

If a new state of one or several real objects appears, another state must have preceded it upon which the new state follows regularly.

• Principle of Sufficient Reason of Knowing

If a judgment is to express a piece of knowledge, it must have a sufficient ground. By virtue of this quality, it receives the predicate true. Truth is therefore the reference of a judgment to something different therefrom.

• Principle of Sufficient Reason of Being

The position of every object in space and the succession of every object in time is conditioned by another object's position in space and succession in time.

• Principle of Sufficient Reason of Acting

Every human decision is the result of an object that necessarily determines the human's will by functioning as a motive.

See also: deterministic system (philosophy).

Laws of thought

Principio de razón suficiente | Principio di ragion sufficiente