〈가〉 The day may come when the rest of the animal creation may acquire those rights which never could have been withholden from them but by the hand of tyranny.
The French have already discovered that the blackness of the skin is no reason why a human being should be abandoned without redress to the caprice of a tormentor.
It may one day come to be recognized that the number of legs, the villosity of the skin, or the termination of the os sacrum are reasons equally insufficient for abandoning a sensitive being to the same fate.
What else is it that should trace the insuperable line?
Is it the faculty of reason, or perhaps the faculty of discourse?
But a full-grown horse or dog is beyond comparison a more rational, as well as a more conversable animal, than an infant of a day or a week or even a month old.
But suppose they were otherwise, what would it avail?
The question is not, Can they reason?
nor Can they talk?
but, Can they suffer?
〈나〉 In this passage, Bentham points to the capacity for suffering as the vital characteristic that entitles a being to equal consideration.
The capacity for suffering ― or more strictly, for suffering and/or enjoyment or happiness ― is not just another characteristic like the capacity for language or for higher mathematics.
Bentham is not saying that those who try to mark ‘the insuperable line’ that determines whether the interests of a being should be considered happen to have selected the wrong characteristic.
〈다〉 The capacity for suffering and enjoying things is a prerequisite for having interests at all, a condition that must be satisfied before we can speak of interests in any meaningful way.
It would be nonsense to say that it was not in the interests of a stone to be kicked along the road by a child.
A stone does not have interests because it cannot suffer.
Nothing that we can 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 they are treated in this way.
〈라〉 If a being suffers, there can be no moral justification for refusing to take that suffering into consideration.
No matter what the nature of the being, the principle of equality requires that the suffering be counted equally with the like suffering ― in so far as rough comparisons can be made ― of any other being.
If a being is not capable of suffering, or of experiencing enjoyment or happiness, there is nothing to be taken into account.
This is why the limit of sentience (using the term as a convenient, if not strictly accurate, shorthand for the capacity to suffer or experience enjoyment or happiness) is the only defensible boundary of concern for the interests of others.
To mark this boundary by some characteristic like intelligence or rationality would be to mark it in an arbitrary way.
Why not choose some other characteristic, like skin colour?