In his interview with Hester Prynne within the prison, Roger Chillingworth declares that he will discover the identity of who is the father of Hester's child, and this man will be his and he will own the man's soul. Then, in Chapter X of The Scarlet Letter, Hawthorne writes that Chillingworth has become the physician of the Reverend Dimmesdale initially to merely to attain the truth; however, as he has proceeded with his investigation, "a terrible fascination" has seized upon the old man, and he has become obsessed.
[He] dug into the poor clergyman's heart, like a miner searching for gold; or, rather, like a sexton delving into a grave, possibly in quest of a jewel that had been buried on the dead man's bosom, but likely to find nothing save mortality and corruption. Alas for his own soul, if these were what he sought!
And, like the miner who seeks the mother lode, Chillingsworth continually probes, questioning Dimmesdale on all sorts of topics, constantly watching the minister, delving deeper and deeper into the soul of the minister. As he questions Dimmesdale, Chillingworth tells himself, "Let us dig a little further in the direction of this vein!" as he seeks the secrets of Dimmesdale's heart. Finally, the physician strikes the vein that contains gold. For, while the minister sleeps, the "miner" pulls aside the vestment of Dimmesdale views that which makes him feels ecstasy: he has discovered the mother-lode.