A hard name for a simple idea
The name sounds scary, but the idea is easy. Overlap means two groups cross each other: some things true of one are also true of the other.
How the argument works
Many people say it's okay to use animals because they lack certain abilities: they don't reason like adults, they don't talk, they don't have a sense of justice, they don't contribute to society.
The argument points out: whatever ability you pick, there will always be humans who don't have it either. For example:
- babies;
- small children;
- people with certain illnesses or who have had serious brain injuries.
These humans also don't reason like adults, don't have a sense of justice, and don't have power over anyone. Just the opposite: like animals, they're completely vulnerable.
The conclusion
Nobody thinks it's okay to do to these humans what is done to animals: raise them for food, use them in tests, or let them suffer without help. If lacking those abilities doesn't justify mistreating these humans, then it doesn't justify mistreating animals either.
An important detail
With vulnerable humans, lacking abilities is usually seen as a reason for more care, not less. A baby needs more protection precisely because it can't defend itself. With animals, that same lack is used as an excuse for less care. It's a double standard.
In short
- For any ability you pick, there are humans who don't have it.
- If that doesn't justify mistreating those humans, it doesn't justify mistreating animals either.
- Vulnerability usually calls for more care, not less.