In the animal rights movement, speciesism is an injustice just as racism or sexism are unjust. The reason racism and sexism are wrong is because race and gender are not appropriate criteria for disparate treatment, which is based on prejudicial beliefs about a certain race or gender. Similarly, speciesism is wrong because one's species is not an appropriate criteria for rights holding. Animal rights activists believe that sentience - the ability to suffer - should be the criteria for rights holding.
More simply, animal rights activists believe that it is wrong to deny rights to non-human animals based on their species, just as it is wrong to treat men and women differently based on their genders. Whether an individual is entitled to basic rights depends on whether that individual will suffer if his rights are violated.
Many people believe that certain species are special because they are highly sensitive or intelligent, such as dogs, dolphins or chimpanzees. Aside from the problematic practice of trying to assign a rating of sensitivity or intelligence to other species, one's intelligence and sensitivity are inappropriate criteria for rights-holding. Exploiting certain animals because they appear less intelligent by human standards is speciesist, because rights holding is determined by sentience and all sentient creatures should have the right to be free of human exploitation and because there is no reason that animals that seem more human-like deserve more rights.
There may be other reasons besides speciesism that cause people to treat different species differently. For example, protecting some animals under the Endangered Species Act is not speciesist because the protection is based not on species but on whether the population is endangered. In that case, the disparate treatment should not be described as speciesist but is still problematic to animal rights activists because the animals should be protected no matter whether their populations are endangered.
The word "speciesist" can refer to someone who engages in speciesism, or can be used to describe behavior that exemplifies speciesism.
Eating beef while protesting dog meat is speciesist.
Once one rejects speciesism, the next logical step is veganism.
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