Animals aren't conscious
The scientific consensus runs the other way. Mammals and birds share the brain structures linked to conscious experience, and they behave in pain the way we do. Whether animals have human-level self-reflection is a separate question, and it is not the one that matters for whether they can suffer.
What makes human life matter morally is not simply being alive. It is a rich inner life: self-awareness, reflection, an understanding of past and future, the ability to know that you are suffering. Animals react to the world, but we cannot be sure there is anyone home in the way there is with us, and their behaviour could be explained by instinct and reflex. If the kind of consciousness that grounds moral worth is distinctively human, then eating animals does not wrong anyone.
The evidence says they are. In 2012 a group of prominent neuroscientists signed the Cambridge Declaration on Consciousness, which concluded that all mammals and birds, and some other animals, have the neurological structures that generate conscious experience. The 2024 New York Declaration on Animal Consciousness went further, calling it at least a realistic possibility for all vertebrates and many invertebrates.
The behaviour fits. Injured animals guard the wound and limp. They will pay a cost to avoid a painful place. Lame chickens given a choice eat more feed laced with painkiller. Animals lose interest in food and play when distressed, and pigs and cows form bonds and show distress at separation. These are the same signs we rely on to judge pain in a human who cannot speak.
It is true that no one can look directly into another mind. But that applies to other people too. We infer their experience from shared biology and behaviour. Applying the same reasoning to animals gives the same answer, at least for the species we farm.
The claim often retreats to 'animals don't have the same level of consciousness as humans.' That may be true for some capacities: planning, language, abstract thought. It is not true for the capacity that matters here. A pig does not need to reflect on its suffering for the suffering to be real to it. Being able to feel pain and fear is enough to make causing them, when it isn't necessary, a moral question.
Using a single word in two different senses within one argument, so that a conclusion appears to follow when it does not.
How it applies here: The argument slides between two meanings of 'conscious'. It asserts that animals are not conscious at all, meaning they have no experience. When challenged, it falls back to claiming they lack human-level consciousness, meaning self-reflection or empathy. The first claim would rule out suffering. The second, even if true, does not, yet it is used as though it carried the same weight.