In most ways, human beings are not equal; and if we seek some characteristic that all of them possess, then this characteristic must be a kind of lowest common denominator, pitched so low that no human being lacks it. The catch is that any such characteristic that is possessed by all human beings will not be possessed only by human beings. For example, all human beings, but not only human beings, are capable of feeling pain; and while only human beings are capable of solving complex mathematical problems, not all humans can do this. So it turns out that in the only sense in which we can truly say, as an assertion of fact, that all humans are equal, at least some members of other species are also "equal" - equal, that is, to some humans.
from the book "" by Peter Singer