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In “Unmoved Mover“, I wrote about the presuppositional argument used by some modern Christian apologists. In this post, I want to say some more about presuppositionalism.

The presuppositionalists have a point in this sense and in this sense only: a worldview is worth being held only if it is possible to reason consistently from that worldview given its own starting principles. If those principles lead inevitably to their own negation, then that worldview is self-contradictory and must be discarded. This is correct as far as it goes. Where presuppositionalists go wrong is in the assertion that Christianity is the only worldview that possesses or could possess this kind of consistency. This assertion is both fantastically arrogant and unequivocally false.

Here’s an example of a worldview that genuinely is inconsistent. The laws of thermodynamics say that, over time, entropy increases to a maximum. The higher-entropy configuration – the more “chaotic” state – is always more likely. Yet our universe as we currently observe it is in a low-entropy state, with plenty of organized pools of energy available to do work.

As we proceed toward the future, the overwhelmingly likely outcome is that entropy will increase. But the laws of physics are time-symmetric: they make no distinction between past and future. Therefore, if we look back into the past, it is also far more likely that entropy was higher back then than it is now. Granted, it’s unlikely that entropy would spontaneously decrease, from a chaotic past to an orderly present. But if we assume that the past had less entropy than the present, we have an even more unlikely configuration to explain – we’re making the problem worse, not better. Applying the laws of thermodynamics in a naive way, then, leads to the conclusion that everything we observe might be a rare, but statistically inevitable, random fluctuation that produces a temporary island of order in the midst of pure chaos.

And the smaller the island of order, the more likely it is that it could arise through random fluctuations in chaos. Thus, compared to the odds of producing an astronomically vast, orderly cosmos, it’s much more probable that random fluctuations would produce a single, isolated observer – a disembodied brain, say – floating in the void of chaos and falsely imagining a whole world surrounding it. This is called a Boltzmann brain.

But there’s a problem. If we are Boltzmann brains, then nothing we believe about the world can be trusted – including the very observations which led us to suspect we might be Boltzmann brains in the first place. The circle of logical contradiction is closed: observations lead us to infer conclusions which in turn lead us to doubt and disbelieve those observations. The Boltzmann-brain worldview falls apart from its own inconsistency. (This is not to say it’s necessarily false – maybe we are Boltzmann brains, there is no way to disprove that – but even if it is true, we could never know it, because the hypothesis itself undercuts all possible basis for believing it.)

The way out of this dilemma is to assume that the evidence is reliable, and that our sensory perceptions and memories of the past reflect a real external world with a real history. This starting point leads to a position which does not contradict itself.

The atheist viewpoint runs along similar lines. Its intrinsic starting point is that the universe is a collection of physical things which exist independently of us, the behavior of which is governed by orderly, immutable principles which we call natural laws. Although the cosmos is complex far exceeding our ability to fully conceptualize it, and although our senses are imperfect and can be misled, we still have the ability to perceive reality with a fair degree of accuracy, to discover its governing principles, and to make inferences about how events will unfold in the future. In other words, we are rational creatures who can learn how the world works.

Contrary to what presuppositionalists claim, this view is consistent. Accepting it as true does not lead to any self-contradiction. (The usual response – that evolution would not produce rational believers – I dealt with in 2006, in “Are Evolved Minds Reliable Truth-Finders?“)

Of course, this by itself does not prove that atheism is true. This is a trivial conclusion, since there are infinitely many consistent worldviews, but only one world. A worldview might be entirely consistent with itself and still be false because it does not reflect the way the world actually is. But self-consistency is the starting hurdle that any worldview must clear before we begin examining it to see whether it corresponds to empirical reality. Atheism is one of the consistent worldviews worthy of consideration, and the attempts of religious apologists to rule it out of hand from the beginning – or to make the ridiculous claim that theirs is the only possible consistent worldview – cannot be sustained.

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DAYLIGHT ATHEISM Adam Lee is an atheist author and speaker from New York City. His previously published books include "Daylight Atheism," "Meta: On God, the Big Questions, and the Just City," and most...