What Is Sufficient Reason? The Demand That Everything Have an Explanation

The principle states that for everything that exists or is true there is a reason why it exists or is true rather than otherwise. It is stronger than the everyday assumption that events have causes, since it applies to facts, to existence and to the way things are, and it admits no exceptions by its own terms. Leibniz made it explicit and treated it as one of the two great principles of reasoning alongside non-contradiction, though the underlying assumption runs through earlier philosophy. Its appeal is that it licenses asking why about anything, which is what enquiry consists of, and that a world where some facts have no explanation would be one where enquiry might simply stop without warning at an arbitrary point. That last consideration is the strongest thing in its favour and is also not an argument that it is true.

The consequences are substantial and several are unwelcome:

• •Cosmological arguments follow directly, since the existence of the universe requires a reason and that reason cannot be within it
• •Contingency becomes difficult, since if everything has a reason then everything follows from prior reasons and could not have been otherwise
• •Free will comes under pressure for the same reason, since an unexplained choice would violate the principle
• •The apparent randomness in quantum mechanics is either explained by something hidden or is a counterexample
• •Brute facts, meaning facts with no explanation, are excluded entirely, and many philosophers think some are unavoidable
• •The principle itself needs a reason on its own terms, which is awkward

Several serious challenges have been pressed. The strongest argues that the principle collapses contingency into necessity, since if every fact has an explanation and explanations are ultimately in terms of necessary truths, then everything follows necessarily and nothing could have been otherwise, which contradicts the obvious appearance that things could have gone differently. Attempts to escape that involve either admitting an unexplained fact, which abandons the principle, or accepting that everything is necessary, which is a substantial cost. A second objection points at quantum phenomena, where individual events appear to be genuinely undetermined, though interpretations differ and the physics does not settle the philosophical question. A third simply asks why the principle should be believed, since it is not self-evident in the way non-contradiction is and no argument for it avoids assuming it.

The principle is frequently read as a claim about causes and it is broader than that, which matters for assessing it. A cause is one kind of reason, appropriate for events, and there are others. Mathematical truths have reasons in the form of proofs without having causes. A fact may be explained by what it consists of rather than by what produced it, as the hardness of a diamond is explained by its lattice rather than caused by it. Purposes count as reasons in some accounts. Conflating these has caused confusion in arguments that invoke the principle, since establishing that every event has a cause is a much weaker claim than that every fact has an explanation, and several arguments slide between the two. Keeping them separate clarifies what is being asserted and frequently reveals that a contested step depends on the slide.

Rejecting the principle has its own costs and the position has to be stated carefully. Nobody sensible abandons the practice of seeking explanations, since it evidently works and the success of enquiry is the strongest practical evidence that reasons are usually available. The limited rejection holds that the principle is a useful regulative assumption guiding enquiry rather than a truth about the world, so one proceeds as though everything has an explanation while accepting that some things may not. That position is comfortable and slightly unstable, since it is unclear how one would recognise a genuinely unexplained fact rather than an unexplained one. A stronger rejection accepts brute facts outright, and on that view the existence of the universe or the values of fundamental constants may simply be so, which many find unsatisfying and none have shown to be impossible.

The principle holds that every fact has a reason it is so rather than otherwise, which licenses asking why about anything. Accepting it delivers cosmological arguments and threatens contingency and free will, since everything would follow necessarily. Rejecting it means accepting brute facts. The middle position treats it as a guide to enquiry rather than a truth, which is comfortable and hard to state precisely.