How does Stevenson show Dr. Lanyon’s shock when he discovers the truth in chapter 9? (PEE)

From the text:

What he told me in the next hour, I cannot bring my mind to set on paper. I saw what I saw, I heard what I heard, and my soul sickened at it; and yet now when that sight has faded from my eyes, I ask myself if I believe it, and I cannot answer. My life is shaken to its roots; sleep has left me; the deadliest terror sits by me at all hours of the day and night; and I feel that my days are numbered, and that I must die; and yet I shall die
incredulous. As for the moral turpitude that man unveiled to me, even with tears of penitence, I can not, even in memory, dwell on it without a start of horror.