The problem of evil is well known: How can there be an all-powerful, all-knowing, perfectly good god if there is evil? The kind of reply most theists give is also familiar to most: Evil is not incompatible with God provided there is some reason for allowing that evil – for instance, in order to prevent something even worse happening. It is only gratuitous evil – evil for which there is no moral justification – that would pose a problem. And most theists appear to be confident that one cannot show that gratuitous evil exists. No matter what example one brings up – whether it’s that of a disease, a war, some natural disaster, or what have you – they think there is some possible explanation as to why God might allow it.
I think it’s actually pretty easy to show that there is gratuitous evil, however.
All the theist has to grant me is that animals sometimes experience pain from which they derive no benefit. Theists are of course free to deny such a thing. But how many would do so (at least before hearing my argument)? It’s obvious that there are many animals that experience pain – their behavior and the similarity of their physiology to our own attest to this fact. It’s also obvious that in many cases, their pain does not serve them any purpose. Pain in general serves a purpose, of course, but an animal that is suffering in its final moments of life isn’t getting anything out of that suffering. In order to gain from it, the animal would both have to have an afterlife and acquire something valuable from its terrible experience – neither of which is at all plausible.
Instead of arguing that the animal gains some benefit in the afterlife, theists are more likely to argue that the animal’s suffering serves some purpose for others. Perhaps without that one animal’s suffering there would be far greater suffering in the future, and not just for animals, but for people. (The animal's behavior might be part of a causal chain that results in a better future.) Or perhaps the suffering is witnessed by a person who then learns compassion from it, thereby bringing about some good that far surpasses the evil experienced by the animal. And so on.
Now, I agree that replies like these are complete BS. But we could argue about them until Jesus returns (or until the cows come home – same thing) and never convince the theist. So instead, let’s consider a very simple fact: that mental experiences are private. The pain that the animal suffers cannot be experienced by anyone else.
That, however, also means that it cannot serve a purpose for anyone else. For God could make it the case that animals only appear to be in pain without actually being so, and the results as far as anyone else is concerned would be the same. Thus, the animal's suffering is not needed to make someone more compassionate: If the animal only appeared to be in pain without actually feeling anything, the person who learns compassion from seeing the animal would be none the wiser. Nor is it needed in order to set up a causal chain of any kind – an animal that behaved the same way without feeling any pain would set up exactly the same chain.
Given that the pain is private, and that it serves no purpose for the animal, it serves no purpose, period. Thus, it is gratuitous suffering – it is completely unjustified. Any theist who believes there is such pain, then, cannot consistently believe in an all-powerful, all-knowing and perfectly good god.