Singer's reproach of discrimination requires greater elucidation.

Discrimination (as it is commonly understood) is a breach of the principle of equality. If two living creatures that are equal in moral terms are treated unequally, this is discriminatory against the one that is disadvantaged. Yet who can be considered equal in moral terms?

The answer to this question depends, in turn, on which qualities of the living creature are recognised as morally relevant. It is taken for granted by Western societies that skin colour, intelligence and sexuality are not morally relevant. The disadvantaging of people on account of their skin colour or gender is therefore considered to be discrimination. But on account of their species?

If, as Singer asserts, the only morally relevant criterion is the capacity of a living creature to suffer, then all living creatures with the same capacity for suffering are in moral terms equal, irrespective of the species to which they belong or what other qualities they may have (for example intelligence, the capacity for language or the capacity for moral action). For Singer this fundamental equality makes the use of animals in the food industry or in biomedical research discriminatory. To quote Singer's own words:

"Racists violate the principle of equality by giving greater weight to the interests of members of their own race when there is a clash between their interests and the interests of those of another race. (...) Similarly those I would call 'speciesists' give greater weight to the interests of members of their own species when there is a clash between their interests and the interests of those of other species. Human speciesists do not accept that pain is as bad when it is felt by pigs or mice as when it is felt by humans".

Singer's approach has been debated from a number of standpoints. Critics particularly point to the fact that man's special moral status is not founded merely in his belonging to a particular species.

1) Moral capacity and intelligence (Bonnie Steinbock)

On the one hand, Singer's approach is criticised for suggesting that the equality of man is to be understood merely as the same capacity to feel. By way of objection, it is argued that within the human species there is an equality across race, gender and sexuality that extends beyond merely the same capacity to feel. As a matter of principle all humans are distinguished by their capacity for moral action - that is, action that takes account of the interests of other living creatures - as well as by their desire for autonomy, dignity and respect.

In contrast to Singer, various authors take the view that such qualities are morally relevant. The higher value attributed to human interests therefore does not constitute discrimination.

2) Equality not as actual equality (Heike Baranzke)

On the other hand, it is pointed out that the equality of man is not founded upon a "set of qualities" shared by all humans, but is instead independent of all qualities. The error of the racist, sexist or heterosexist would not then be that he or she attributes greater weight to a morally irrelevant inequality (skin colour, gender, sexuality) than to the more significant and exclusively morally relevant equality (intelligence, capacity for moral action). Rather, the racist, sexist or heterosexist commits the error of making the "value" of a person dependent in any way on his or her qualities. To put it another way, the core of the idea of equality is not descriptive, but normative.

In light of this critique it is difficult to specify criteria that creatures other than human ones would have to meet if they were to be deserving of respect in the same way.