The idea being debated
Some people argue: there's nothing wrong with killing an animal, as long as it had a good life, is killed without pain, and is replaced by another animal that will have an equally good life. Animals would be "replaceable," like parts.
The reality
The first answer is about the facts: animals raised for consumption live with a lot of suffering and are killed in brutal ways. The ones born to replace them go into the same suffering. So the idea doesn't describe what actually happens.
But what about in theory?
The author goes further and asks: why would anyone think one animal can be swapped for another?
The usual explanation is based on the idea of identity: what makes you who you are over time would be your psychological connectedness (memories, desires, plans). If an animal had little psychological connectedness, it wouldn't matter whether you kept that animal alive or swapped it for another: "it" wouldn't really keep existing anyway.
Criticism 1
Even if this reasoning were right, it would only apply to beings with no psychological connectedness at all. And almost no animal is like that, maybe none.
Criticism 2: what makes you who you are?
The author uses a thought experiment. Imagine all your memories and your personality are copied into a new body. Meanwhile, your original body stays alive but loses all its memories.
Where would you be? It seems that you stay in the original body: it's your consciousness that will feel everything that happens to it, even without remembering the past. The copy would be another person, very similar to you.
If that's right, each animal is a unique individual. Killing it and having another one born isn't swapping a part: it's ending someone and creating someone else.
In short
- In practice, "replacement" animals suffer the same as the ones before them.
- The idea of replacement depends on a debatable view of identity.
- Each animal is a unique someone, and killing it ends that someone.