The idea being debated
Someone could reply to the last chapter like this: "Okay, a baby can't reason yet. But it has the potential to reason when it grows up. Animals don't have that potential."
According to this idea, all humans would have this potential, and only humans. So it would be okay to use animals as food, clothing, or test subjects, but not humans.
Problem 1: not every human has that potential
Some people have illnesses or injuries that mean they will never develop these abilities, not even in the future. Still, nobody thinks it's okay to mistreat them.
Sometimes people who defend the argument reply: "But science might advance, or a miracle might happen." But if that "maybe someday" counts, it could count for animals too. Science could someday boost a pig's intelligence. The argument loses its force.
Problem 2: it doesn't explain why it would matter
Even if someone has the potential to reason, why would that make it wrong to make them suffer today? The argument doesn't explain.
Problem 3: the real reason is something else
Think of a small child. Why is it wrong to make them suffer? It's not because they'll grow into a smart adult. It's because suffering is bad, right now, for them.
If it were about potential, the suffering of a child with a terminal illness, who won't grow up, wouldn't matter. Almost nobody accepts that.
In short
- Not every human has the potential to develop reason or language.
- The argument doesn't explain why a future potential would change how much a present pain counts.
- Suffering is bad in itself, for any being that can feel.