The idea being debated
Another common saying: "Humans invented ethics to protect themselves. People respect those who can get back at them. Since animals can't fight back, we don't need to respect them."
Problem 1: this wouldn't justify treating only animals badly
If ethics were only about protecting those who can fight back, babies, frail elderly people, and very sick people would also be left unprotected. Nobody accepts that. So the argument doesn't work for separating humans from animals.
Problem 2: that's not how things really are
It isn't true that people only respect those they're afraid of. Lots of people help others who will never be able to pay them back: they donate blood, adopt animals, take care of strangers. People also care about the weak.
What if it's about where ethics came from?
Maybe the argument means something else: that ethics started, way back in the past, as a way to protect ourselves.
Even so, there are two problems. First, this is probably false too: cooperation and caring for others seem to have been there from the start.
Second, and more important: telling how something started doesn't tell you whether it's right. Medicine may have started with rituals and superstitions, and today we can tell a good treatment from a bad one. Where an idea came from is a question for history. Knowing what's fair is a different question.
In short
- If ethics were only mutual protection, many humans would be left unprotected too.
- People don't only respect those they fear.
- Where ethics came from doesn't decide what's fair today.