In the necessity with which one follows

The protagonist, Philip moved in with his uncle Mr. Carey, the Vicar of Blackstable after his mother's death.
He goes to the medical school St. Luke's in London.
He met his friend and a painter Lawson at London; they had studied together in Paris.
Lawson tells Philip about their acquaintance, the middle-aged poet Cronshaw.
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"~A little while ago he started on whiskey again. He said he was too old to turn over any new leaves. He would rather be happy for six months and die at the end of it than linger on for five years. And then I think he's been awfully hard up lately. You see, he didn't earn anything while he was ill, and the slut he lives with has been giving him a rotten time."
"I remember, the first time I saw him I admired him awfully," said Philip. "I thought he was wonderful. It is sickening that vulgar, middle-class virtue should pay."
"Of course he was a rotter. He was bound to end in the gutter sooner or later," said Lawson.
Philip was hurt because Lawson would not see the pity of it. Of course it was cause and effect, but in the necessity with which one follows the other lay all tragedy of life.
[Of Human Bondage by Somerset Maugham]
I'd like to know what "in the necessity with which one follows" means.
Thank you in advance for your help.