Principle of Sufficient Reason

Lucas is an atheist apologist who doesn’t like the principle of sufficient reason.

This idea that the universe needs “Sufficient Reason” is, to me, totally BS. What “reason” does a tree have? What reason do “I” have? Some things just “are”.

In this post, I’d like to explain what I understand of the principle of sufficient reason, how it could apply to trees, himself, and why (IMHO) it doesn’t necessarily lead to infinite regress. I apologise in advance that I’m not a philosopher, so (although this is the way I see things) sorry if I have butchered other people’s arguments.

On Being Rational

The Principle of Sufficient Reason (PSR) says that “For every fact F, there must be an explanation why F is the case.”

To think that PSR is true, is simply what it means to be rational. There is a reason, even if we don’t know what it is. It’s this principle which underpins science, and motivates us look for those reasons. The principle I use for every question, not just when I’m thinking about science or God: For every fact there is an explanation or a reason. That’s what it is to be rational.

Of course, you don’t have to accept reason - you don’t have to be rational. And if you reject reason, there’s no argument I could ever offer you. How could you reason with someone who rejects reason? You can’t.

Lucas asks,

What reason does a tree have?

The principle of sufficient reason says that for any fact about the tree, there is a reason why that’s the case.

Take, for example, the apricot tree growning in my back yard. There is a reason why it is growing there. A seed was planted in a nursery, and that seed grew into a small tree, at which point it was bought by a couple of homeowners, and planted in their garden. There it was cared for, and grows there until today.

What reason do “I” have?

Not to be rude, but when a mummy and a daddy love each other very much…

Contingent and Necessary

One important thing to realize is that there are two different types of reasons that can be given:

Therefore we must assign some positive cause, or reason, why [a thing] exists— either an external one, i.e., one outside the thing itself, or an internal one, one comprehended in the nature and definition of the existing thing itself.

I would call things of the first type ‘contingent’ - they depend on things outside themselves. And things of the second type ‘necessary’ - their reason for existing is bound up in the thing itself, not something external.

Lucas' examples, of a tree and of himself, are of the first type. There’s no reason intrinsic to either why they have to exist. In reality they demonstrably depend on other things. Physics, botany, biology and chemistry describe many of these dependencies.

Which brings us to a pretty basic point: You can’t just arbitrarily ascribe being necessary to anything you like. You can’t just declare that trees (or other physical objects) are necessary, any more than you can say that green objects are actually blue, up is down, or that the current West Indes cricket team is actually good.

So how do we avoid infinite regress? If someone thought there were only contingent things- that would lead to an infinite regress. No matter how far we’d explained, we’d still have to explain further. It wouldn’t even matter how long the chain was, we would still have to add more, and there would still be no ultimate justification for existence. Only if the chain led back to something necessary, we could validly terminate it.

To be well justified, ultimately something necessary must exist.


I’m far from a philosopher, but describing the Principle of Sufficient Reason - the idea that there are rational explanations for facts as “totally BS” is arrogant. It’s this idea which motivates science, and this idea which distinguishes between someone who is rational and someone who is not.

I introduced being necessary and contingent. While Lucas might want to say physical objects (like trees and leaves) are brute facts not requiring an explanation, in reality that a plant exists is a contingent fact. A plant depends on seed, sunlight, water, fertilizer, good soil, not to mention a multi-million year long evolutionary pathway for its existence.

Finally we reasoned that having only contingent things alone lead to infinite regress, and so to avoid that absurdity, something necessary must exist to terminate it.

That’s a lot to write in one waffle. I hope I haven’t butchered the philosophy too badly. Seriously, the thing to do is to go and read. A lot of this stuff, argued infinitely more rigorously than some amateur hack like me does for fun, is at your fingertips. Just google, and you can read what some of the greatest minds mankind has ever produced (like Leibniz, Spinoza and Kant) have to say.