The Unpardonable Sin is one "for which Heaven could afford no mercy." Ethan Brand says, with pride, that it is a sin that grew inside his own heart, that it is the "sin of an intellect that triumphed over the sense of brotherhood with man and reverence for God." Although Brand knows that he will be repaid with "immortal agony" for his committal of this sin, he declares that he would commit it again and incur the guilt all over if given the chance. When Brand is approached by a little old man whose daughter had run off with the circus, the man asks Brand if he has seen the daughter, who is supposed to be quite famous. Now, Brand's eye "quailed beneath the old man's." The daughter, Esther, of whom the old man seeks news:
was the Esther of our tale, the very girl whom, with such cold and remorseless purpose, Ethan Brand had made the subject of a psychological experiment, and wasted, absorbed, and perhaps annihilated her soul in the process.
Later, as Brand muses alone, the narrator reveals that he used to love his fellow man and that the heart is a "temple originally divine" and ought to be respected by others, even if we each desecrate our own. He never wanted to learn the secret of the Unpardonable Sin, but then, over time, he began to grow his intellect and he found that his heart "withered" in his chest and "ceased to partake of the universal throb." Brand became a "cold observer" who experimented with men and women, making them "his puppets" so that he could study them.
Thus, Ethan Brand became a fiend. He began to be so from the moment that his moral nature had ceased to keep the pace of improvement with his intellect." Therefore, it seems that the Unpardonable Sin is the loss of love for one's fellow human beings and the treatment of them as though they are less valuable and worthy of love than one is. Denying the humanity of others is the worst sin possible.