Ethics (or moral philosophy: the two are synonims, the only difference being that the former is a word of Greek origin, the latter of Latin) is the rational exploration of what is right and wrong, what ought to be and what we ought to do.
Any moral theory must have a theory of moral status, defining moral agents (beings who act on moral grounds) and moral patients.
Not all beings are moral patients, ie not all beings should be considered when we make moral decisions. Some beings, like inanimate objects, don’t pertain to the moral sphere because they don’t have the characteristics that would make them affected by an action in a subjective sense: in a word, they experience nothing. They, therefore, have no interests.
So, what beings belong to the sphere of moral concern and why?
Animals, both human and non-human, both as individuals and species, possess a very high number of characteristics.
The overwhelming majority of these characteristics have no effect on the moral status of their owner: having a certain hair colour, or thickness of fur, or being able to fly, walk or swim, are examples of such characteristics.
What characteristics are relevant to ethics, then?
Although there is no absolute agreement on this among moral philosophers, there are several characteristics which are generally recognised as the likely candidates.
Moral philosophers may disagree on their specific lists of characteristics, but most characteristics will appear on most philosophers’ lists. That is: the school of moral philosophy A, utilitarianism for instance, will give more importance to sentience, whereas the school of thought B might give more importance to rationality. But basically, the characteristics of moral relevance are a circumscribed number.
Sentience will appear on almost every philosopher’s list. Sentience is defined as awareness of sensation and the ability to feel pleasure and pain.
Other candidate characteristics include memory, self-consciousness, oral language, a sense of justice, intelligence, ability to communicate, concern for others, playfulness.
Almost all these characteristics are variable. Different individuals have them in various degrees.
Human beings, too, greatly differ in their possession of them. Some human beings don’t possess some of them at all.
Newborn, mentally retarded, severely senile, brain-damaged humans fall into this category.
On the other hand, many capacities that have been proposed as a demarcation line between humans and non-humans have turned out, on closer scrutiny and as our knowledge of animals progresses, not to be unequivocally unique to humans.
Many other animals possess them in some degree. Examples of these behaviours and characteristics are the development of complex family ties, a system of morality, advanced social rules, problem solution, the expression of emotions, wars, sex for pleasure, abstract thought.
If we wish to restrict our definition of what is necessary 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.