To be a castaway is to be a point perpetually at the center of a circle. [...]. When you look up, you sometimes wonder if [...] there isn't another one like you also looking up, also trapped by geometry, also struggling with fear, rage, madness, apathy. (2.78.5)
Pi describes the feeling, at sea, of being the absolute center. No matter where he is, the distance to the horizon remains the same. But the phrase "perpetually at the centre," for Pi, also suggests loneliness and spiritual abandonment. And such utter and extreme isolation, for Pi and anyone else, leads to madness.