It's typically presented as a syllogism. This a logical argument that used two related premises to reach a conclusion.
- P1: Whatever begins to exist has a cause;
- P2: The universe began to exist;
- C: The universe has a cause
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As the argument does not include any deities, it is imputed that this cause, must itself be uncaused. Conveniently theists knew what this cause was all along. This is their favourite deity, which by fiat is eternal, existing outside time and space and, uncaused.
The popularity of the syllogism doesn't disguise its flaws. It is these flaws that have prevented a stampede of atheists toward Christianity. Let me elaborate.
A syllogism depends on the its premises being correct. It is not good enough for them to be possibly correct. And there are reasons to suspect the premises are not.
Let's look at the first.
How do we know that everything that begins to exist has a cause?
- The reason we have the qualification "begins to exist" is to exclude deities with eternal properties. If it was just 'exists' then the syllogism would net in gods too. In short, it is a special pleading fallacy introduced right at the start. How do we know deities are able to exist without a beginning? We don't. No evidence is attached to prove this. It's just one more thing we have to believe about gods on faith alone.
- It's an uncertain premise. From what we understand about quantum mechanics, the quantum world behaves stochastically. It's a random world at that level. Negative and positive sub-atomic particles wink in and out of existence.
- The above means that P1 is not self-evident
- Induction is not strong enough to prove that P1 is correct. We've sampled a tiny fraction of the universe and our observed sample size of universes is still stuck at 1.
- It makes the term 'cause' do a lot of work. Here the syllogism tries to conflate causality (in the sense of purposely caused by an agent) with other causality in other contexts (e.g. occurs naturally without intercession because of environment and natural laws). As a term, causes are not really part of the conceptual toolbox of Fundamental Physics. I'm not convinced at this level, cause is an appropriate term.
- It is also a category fallacy. Universes don't fall into the same category as phenomenon within the universe.
- Similarly, it's not clear what we mean by 'begins to exist'. If time is an emergent property of the universe, then it becomes difficult to talk about a point where the universe 'begins to exist' (see the Hawking-Hartle no-boundary condition).
How do we know the universe began to exist?
We don't. The standard model of the universe converges to a singularity, where the regular laws of physics as we understand them don't apply. This could be a genuine beginning, or it could be just the observable part of other ways the universe evolved.
Alternatives include a bouncing universe (our universe originated on another one that collapsed, before bouncing back from a singularity), cyclical (the various ekpyrotic models), reproducing from others etc.
P2 has yet to be proven to be true.
Does the Universe Have Cause?
In the sense that some event occurred that initiated an inflationary period for the universe? That seems plausible. For instance, some quantum event is potentially able to cause this. Does that mean we know that this is how it happened? No. But there are natural explanations. We don't have to invoke gods, and I'm not sure why we would. What falsifiable hypotheses could we test? What predictions does this make? The scientific model predicts a flat, homogeneous universe with Cosmic Background Microwave radiation. The divine model predicts a bloodgod who is outwitted by a talking snake, so that he has to mate with a virgin to kill his own offspring because two people ate some fruit. It's not a good way to build credence.
Gods have a problem as they're not mentioned in the syllogism, and as the syllogism is attempting to prove gods exist, it's not an independent proof of said gods. We do know say, that quantum perturbations do happen. The syllogism for gods starts becoming a little circular at this point.
This lack of evidence for gods makes the whole syllogism unconvincing.
Summary: Syllogisms aren't Evidence
Syllogisms are interesting logical arguments. We can say if the premises are true or not, and we can say whether the conclusion follows from these premises. But they're not evidence, they're either logically true or false. In this case, the Kalam or first cause argument suffers a plethora of philosophical and scientific flaws.
Let me conclude with my own syllogism to illustrate this point:
P1: Everything that exists has a natural cause (by induction)*
P2: Gods are supernatural
C: Gods cannot exist or cause anything that does exist.
* E.g. We've replaced many supernatural explanations with natural, and never a natural explanation with a supernatural.