Sufficient Reason, Principle of Sufficient Reason (Metaphysics)

Sufficient reason, Principle of Sufficient Reason . This is the principle on which every contingent truth is grounded. It is neither less fundamental, nor less universal than that of noncontradiction . Everyone complies with it by nature, for there is no one who resolves to do one thing rather than another, without there being a sufficient reason that makes them realise that the former is preferable to the latter.

When we ask someone to account for their actions, we press them with questions until we manage to uncover a satisfactory reason, and we always feel that we cannot possibly force our minds to admit of something without a sufficient reason , that is to say, without a reason that allows us to understand why that thing is the way it is, rather than otherwise.

If one were to attempt to deny this great principle one would fall into strange contradictions: for once one admits that something may happen in the absence of a sufficient reason , one can no longer guarantee that one thing is the same as it was the moment before, for that thing could turn at any moment into something else of another kind; we would thus be left with nothing but momentary truths.

I can guarantee, for example, that everything in my room is still the way I left it, because I know for sure that no one entered my room since I left; but if the principle of sufficient reason does not hold, my certainty turns into a fantasy, for everything in my room could have gone topsy-turvy without anyone capable of disturbing it having entered.

Without this principle, nothing can be identical, for two things are identical when one of them can be replaced by the other without any change occurring with respect to the property under consideration. Thus, for example, if I have a ball of stone and a ball of lead, and if I can place one rather than the other in the pan of a balance without altering in any way the balance’s position, I say that the weight of those balls is identical, that it is the same, and that they are identical in weight. However, if anything could happen in the absence of a sufficient reason , I could not declare the weight of the balls to be identical, even at the very moment when I assert it to be identical, since, for no reason, a change might well occur in one which might not occur in the other, and therefore their weight would not be identical, which is contrary to the definition.

Without the principle of sufficient reason , you will no longer be able to assert that the universe, every part of which is so perfectly connected to the others, could have been produced only by supreme wisdom. For it could arguably all have been the product of chance – that is, of nothing – were it possible for an effect to be brought about in the absence of a sufficient reason . What sometimes happens in dreams may give us some notion of a fantastical world in which all events take place without a sufficient reason . I dream that I am sitting in my room, busily writing; all of a sudden, my chair turns into a winged horse, and the next moment I find myself hundreds of miles away from the place where I was, in the company of long-dead people. None of this could happen in this world, for there would be no sufficient reason for any of these effects. It is this principle that distinguishes dreams from wakefulness and the real world from the fantastical one depicted in fairy tales.

In geometry, where all truths are necessary, only the Principle of Noncontradiction is used. But when it is possible for a thing to exist in different states, I cannot be sure that it is in one state rather than another, unless I provide a reason for what I assert. Thus, for example, I can be sitting, lying down, or standing: all these states are equally possible. Yet if I am standing, there must be a sufficient reason for me to be standing, rather than sitting or lying down.

Applying geometry to mechanics, Archimedes clearly recognised the need for sufficient reason , since, determined to demonstrate that a scale with arms of equal length will remain perfectly balanced when equal weights are placed on each arm, he showed that, given that both arms and both weights were identical, the balance had to remain at rest because there was no sufficient reason for one of the two arms, rather than for the other, to go down. Mr Leibniz, who was intent on discovering the sources of our reasoning, seized on this principle and perfected it. He was the first to enunciate it explicitly and he introduced it into the sciences.

The principle of sufficient reason , moreover, is the ultimate basis of rules and customs, which are based solely on what is called convention ; for the same people can act according to different customs; they can determine their actions in several ways; and any time one chooses, amongst many possible actions, the one which has more reason on its side, the action becomes good and cannot be blameworthy. But one terms an action unreasonable whenever there are sufficient reasons not to commit it. And it is precisely in virtue of these principles that one can declare that a certain custom is better than another, that is to say, that it has more reason on its side.

This principle banishes from philosophy all scholastic arguments. For although the Scholastics did indeed admit that nothing happens without a cause, they nonetheless put forward as causes plastic natures, vegetative souls, and other such words completely devoid of meaning. But once it has been established that a cause is good only inasmuch as it satisfies the principle of sufficient reason , i.e., inasmuch as it contains something by means of which one can show how and why the effect can be produced; then one can no longer be content with those big words that were used instead of ideas.

For example, when one is explaining why plants germinate, grow, and survive, and one gives as the cause of those effects a vegetative soul that can be found in all plants, one is indeed advancing a cause for those effects. Yet it is a cause that cannot be accepted because it contains nothing that allows me to understand how vegetation actually takes place. For even if I posit such a vegetative soul, it in no way helps me to understand why it is that the plant that I am examining has a certain structure rather than another, nor how that soul can lend shape to a machine like that of the plant. [1]

All this notwithstanding, it is possible to make a sort of argument ad hominem against the principle of sufficient reason by asking Mr Leibniz and Mr Wolff how they can reconcile it with the contingency of the universe. In fact, contingency presupposes indifference of equilibrium. [2] Now, what could be more contrary to indifference of equilibrium than the principle of sufficient reason ? One must therefore say that the world exists, not contingently, but rather by virtue of a sufficient reason , and admitting this could bring us to the verge of Spinozism. It is true that both Leibniz and Wolff attempt to escape this difficulty by defining contingency simply as something whose opposite is not impossible. But it is nonetheless true that the principle of sufficient reason does not leave contingency intact. The more reasons there are calling for the existence of one plan, the less the others become possible, that is, the less they can lay claim to existence.

Nevertheless, the principle of sufficient reason is extremely useful. Most fallacious arguments arise solely because this maxim has been overlooked. It is the only thread that can guide us through the labyrinths of error that humans have built up for themselves, only to enjoy the pleasure of getting lost. We should therefore admit nothing that violates this fundamental maxim, which reins in the innumerable vagaries to which the imagination is given whenever it is not subjected to the rules of strict reason.

1. From the beginning up to this point the article is copied with only minor changes from Emilie Du Châtelet’s Institutions de Physique (Paris: 1740), an English translation of which has appeared in 2009. See Emilie Du Châtelet, Selected philosophical and scientific writings , ed. by J. P. Zinseer and I. Bour, Chicago-London: University of Chicago Press, 2009. For more on Emilie Du Châtelet and the Principle of Sufficient Reason see P. Veatch Moriarty, The Principle of Sufficient Reason in Du Châtelet’s Institutions, in SVEC , 2006:01, Oxford: Voltaire Foundation, p. 203-225.

2. The original text has “différence”. When revising part of this article for the Encyclopédie méthodique , Jacques-André Naigeon corrected this sentence to read “La contingence en effet suppose une in différence d’équilibre” (italics mine). See J.-A. Naigeon, Encyclopédie méthodique , Paris: Agasse, 1794, vol. 3, p. 596.