Singer gives a very basic example: "A stone does not have interests because it cannot suffer. Nothing that we could do to it could possibly make any difference to its welfare. A mouse, on the other hand, does have an interest in not being tormented, because it will suffer if it is" . What can be seen here is the idea of every sentient being, even a mouse, as possessing a capacity to have interests. Pushing further, potential interests of a being hinge on its ability to experience both pleasure and pain. Without digressing too much, the interests of various sentient beings can be considered equally, unless there is some condition or reason which denies equal moral consideration. And this is precisely the goal of this essay, to determine if species difference is morally relevant. The capacities which Singer identifies explicitly limit the concern here to sentient beings which will exclude plants and organisms which naturally lack some form of consciousness.
In evaluating speciesism as a coherent charge to levy against much of our society, most critics of Singer have attacked the idea solely on utilitarian and consequentialist grounds. While this may be the correct approach to attack the ideology, it in some ways seems like an escape for such critics to avoid taking on what may be the most vital element of speciesism, namely the element of prejudice and bigotry with which it can possibly be attached. This will explain why the investigation here will leave others to examine Singer's utilitarian ideas, and will focus on the assertion of inherent bigotry in speciesism. Along with this focus which is relevant is the objection of marginal cases, and how pro-speciesists may try to deal with such objections and if in fact they can be successful in its defense.
Perhaps the best and most common analogy by which to better understand speciesism is to equate it with various other prejudices, most notably racism and sexism.
