Claim: You can't prove that anything is real so animals might not have rights

Details

Various possibilities:

• some form of solipsism might be right
• we are potentially in a simulation
• the brains of animals or others specifically aren't real
• consciousness is an illusion

And therefore, we can't be sure that eating animals is objectionable.

Problems With This Argument

1. This is a shifting of the burden of proof

All of these ideas are unfalsifiable. Therefore, one would need to demonstrate their truth because they are indistinguishable from any other claim that is just asserted and can't be distinguished from nonsense. We can invent an infinite set of such claims, "The sky is actually red but it just appears blue to you because you're under mind control".

Unless you actually can demonstrate such claims, then to suggest that animals have no rights based on some hypothesis is just another baseless assertion.

2. We recognize this as a bad moral argument in any other context

"Oh, you know, sorry I stole your car. Reality is probably an illusion. Also, I'm not sure that you're conscious" is a bad argument for doing anything unethical. It certainly would be a poor argument for doing anything to another person, even if they were an infant or suffered from a mental handicap. So it would be a poor defense of animal abuse as well.