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The first signpost comes one day when Jack walks up the hill to the highway where he drops off a letter to his mother, asking her to give a kiss to his cat, and also a letter to his buddy, Julien.
He passes Alf on the way back and feels "strangely low, as tho premonition of the next day."
He tries taking a deep breath but it leaves him feeling faint, as though he's recognized "the form of horror of an eternal condition of sick mortality in[him]—In [him] and in everyone."
Seeing himself as "doomed, pitiful," he has no idea what to do – how can someone go and chop wood after feeling like that? So he sits and stares at the sea, which seems to yell at him to go instead of sitting around and moping.
So Jack runs away from the seashore, and he never comes back without "that secret knowledge: that it [the sea] didn't want [him] there."
That, says Jack, was the first indication of his coming "flip."
When it's time for him to go back to San Francisco, he takes all the food he has left and spreads it on a table outside for the animals to have.
Only then does he realize that this isn't his cabin, and that he doesn't have the right to let animals and rats roam free in it. So to fix the situation, he leaves the rat poison uncovered on the top shelf.