system
Given the following post, respond as if you're commenting on Reddit:
human
How do you explain to a person 200 years ago that racism is bad?

Recently in a practical ethics class ( not a major, was just interested ), the topic of animal rights was brought up, naturally, Peter Singer's Animal Liberation was a must-read, and "speciesism" was compared with racism and sexism and called a mere prejudice, instinctively as a meat lover but less of an animal abuse lover, it is hard to accept Singer's stance. Shelly Kagan's What's wrong with Speciesism points out that Singer, too, resorted to instincts in proposing only sentient agents have moral standing.

Shelly Kagan "defended" a view that ( probably ) most of us agree on with the concept of "Modal Personism", but seem to still resort to instincts. While it sounds very nice and convenient for us, it gets me wondering, a person 200 years ago might be able to use a very similar argument to defend racism, and one from our times would find it very hard to accept, then how can one consistently accept Kagan's defence?

I still love my meat, but this remains one problem I find hard to be consistent with, I can just go ahead and be a proud speciesist, or maybe take Carl Cohen's view and say we have a unique responsibility to fellow humans, but it still feels like prejudices Singer suggested.

Is there an actual way to differentiate racism from speciesism, or maybe even sentientism? Or is it in the end really just instincts and culture?
gpt
It'd be pretty trivial to convince some 200 years ago that 'In the case of peoples which have no significant difference between them, we should not treat them differently.' The problem would of course be that 200 years ago that people did think there were significant differences between races, something that a lot of people still believe to this day. 

>Is there an actual way to differentiate racism from speciesism, or maybe even sentientism? 

The traditional defence of 'speciesism' is that there are in fact (morally) significant differences sufficiently strong between us and most animals, such that its permissible for us to use them in basically whatever way we want. The question of whether this is actually true or not is one of the major fulcrums around which discussions of animal rights go around in Philosophy, probably the biggest one.