Dr. Jekyll writes this idea in his memoir of his terrible experience, which is recorded in Chapter 10: "Henry Jekyll's Full Statement of the Case." It is best understood from within the context of Jekyll's record. At the beginning of his memoir, he explains that he has always keenly felt the dichotomy within himself of a high nature and a low nature; a nature that was dedicated to the "futherance of knowledge or the relief of sorrow and suffering" and a nature that "laid aside restraint and plunged in shame." He explains that this "profound duplicity," this duality, this division between the powers of "good and evil" that he was aware of and indeed cultivated, caused him to hide his evil "pleasures" with an "almost morbid shame." It is in the context of this discussion of "evil ... pleasures" that Jekyll writes:
I was driven to reflect deeply and inveterately on that hard law of life, which lies at the root of religion and is one of the most plentiful springs of distress.
The three clues that lead to understanding this expression are "root of religion" and "plentiful springs" and "distress." If evil behavior of which a moral man is "morbidly ashamed" (morbidly here means unwholesomely, extremely) is juxtaposed with these clues, then distress caused by the root of religious doctrine that ever springs up leads straight to guilt. Jekyll was compelled to "reflect deeply" as an habitual behavior upon the "hard law of life" that is guilt. This is confirmed by his next sentences where he confesses to be a "profound double-dealer," in other words, one who shows one lifestyle to the world in "the eye of day" while guiltily hiding quite another lifestyle "plunged in shame" and the dark cover of night. In summary, "the hard law of life being one of the most plentiful springs of distress" means guilt and unstoppable feelings of guilt.