As you might guess, the essay is a send-up of the idea that humans are the "highest" of the animals, or the highest of earthly beings on the Great Chain of Being. By contrast with other animals, Twain finds us avaricious, quarrelsome and cruel, and not nearly as "reasoning" as we like to think. ("Man is the Reasoning Animal. Such is the claim. I think it is open to dispute....His record is the fantastic record of a maniac.")
Twain traces our various questionable characteristics to the ultimate Defect of our being endowed with "The Moral Sense":
He is the only animal that has it. It is the secret of his degradation. It is the quality which enables him to do wrong. It has no other office. It is incapable of performing any other function. It could never have been intended to perform any other. Without it, man could do no wrong...Twain goes on to compare the moral sense to having rabies, to suggest that it, too, is a disease, but a worse one:
Rabies is bad, but it is not so bad as this disease. Rabies enables a man to do a thing which he could not do when in a healthy state: kill his neighbor with a poisonous bite. No one is the better man for having rabies. The Moral Sense enables a man to do wrong. It enables him to do wrong in a thousand ways. Rabies is an innocent disease, compared to the Moral Sense. No one, then, can be the better man for having the Moral Sense.Is this a sound argument? One could accuse Twain of ignoring the counterpoint that the "moral sense" also enables one to do good. Nevertheless, he concludes:
What, now, do we find the Primal Curse to have been? Plainly what it was in the beginning: the infliction upon man of the Moral Sense; the ability to distinguish good from evil; and with it, necessarily, the ability to do evil; for there can be no evil act without the presence of consciousness of it in the doer of it.Unless Twain thinks there's an asymmetry between doing good and doing evil, it seems like we should say the same thing about doing good--that there can be no good act "without the presence of consciousness of it in the doer of it." Twain certainly doesn't appear to be an internalist about moral motivation; that seems to be implicit. So, the good is not internally motivating. He suggests that our ability to distinguish good and evil enables us to consciously choose evil. Given his inattention to the converse point--that we could also consciously choose to do good--I take it that he finds the converse point to be of little consolation.
Twain's point may simply be that the "moral sense" alone does not make us good, or "better" than other animals. The ability to distinguish between good and evil, in the way Twain characterizes it, is itself a morally neutral capacity, like sight or echolocation. Sure, we have the capacity to do good, but we also have the capacity to do great evil. Animals, by contrast, are not "vicious" (as I have discussed before), though animals do engage in violence and killing. The difference, for Twain, appears to be that they do not choose it for its own sake, nor do they rationalize their violence by dehumanizing their victims, or by claiming metaphysical/theological license for their acts. Animals do not make excuses. I wonder whether anyone has made that suggestion: that what distinguishes humans from other animals is that we make excuses. (If not, then you heard it here first!)
Or, to follow Twain's form: humans are the only animals that can apologize, or need to.