Let's start with a distinction: what is the difference between animal welfare (or animal protection) and animal rights (or animal liberation)?
The traditional societies for the prevention of cruelty to animals of the past accepted that the welfare of nonhuman animals deserves protection only when human interests are not at stake. Human beings were seen as belonging to a separate moral domain, with a gulf between them and all other animal species, and infinitely superior to them.
If our interests conflict with theirs, it is always their interests which have to give way.
The new position puts human and nonhuman animals on the same moral footing.
This is what we mean when we say, as Peter Singer does in his book Animal Liberation, that "all animals are equal."
We come to the crucial point of a demarcation line in ethics.
What defines and bounds the beings to which we apply moral judgements, in contrast to the ones outside the scope of moral considerations?
This ethical demarcation line cannot reside in a biological factor per se. Membership of a species is one such biological factor; others are membership of a race or sex.
One of the reasons why biological factors are a dangerous demarcation line is that they annul individual differences: the actual characteristics possessed by the particular individual we are dealing with become irrelevant, and the individual is assimilated into the group to which it belongs.
The basic ethical position concerning animals boils down to what follows.
There is no morally relevant characteristic which is possessed by ALL human beings AND ONLY by human beings.
If we wish to restrict our definition of what is necessary to possess to be included in the sphere of moral concern to higher characteristics, such as self-consciousness, oral language or a sense of justice, then not all human beings possess them, so some human beings will be excluded from the moral sphere of consideration.
If, on the other hand, we decide to broaden our definition so as to admit characteristics like capability of feeling pain, then non-human animals must be included into the moral sphere too.
There is no way to escape this iron logic.
This is what in philosohy of science would be called a hypothetic-deductive system, of the same kind as Euclidean geometry, but even more solid than Euclid’s system which, after all, could be joined by 19th-century non-Euclidean geometries that served as the basis for Einstein’s relativity theory.
In the case of anti-speciesist logic, on the other hand, there is no way to replace any postulate, as it happened with Euclid's fifth postulate or Parallel Postulate, whose replacement offered the possibility of an alternative system.
Anti-speciesism is one of the strongest types of deductive thories that we have, it has the same logical power as 2+2=4.