Once you stumble... human nature is on you.

No passion is stronger in the breast of a man than the desire to make others believe as he believes. Nothing so cuts at the root of his happiness and fills him with rage as the sense that another rates low what he prizes high.

To whom can I expose the urgency of my own passion?…There is nobody—here among these grey arches, and moaning pigeons, and cheerful games and tradition and emulation, all so skilfully organised to prevent feeling alone.

No passion is stronger in the breast of man than the desire to make others believe as he believes. Nothing so cuts at the root of his happiness and fills him with rage as the sense that another rates low what he prizes high. (...) It is not love of truth but desire to prevail that sets quarter against quarter and makes parish desire the downfall of parish. Each seeks peace of mind and subserviency rather than the triumph of truth and the exaltation of virtue.

Once you fall, Septimus repeated to himself, human nature is on you. Holmes and Bradshaw are on you. They scour the desert. They fly screaming into the wilderness. The rack and the thumbscrew are applied. Human nature is remorseless.