Coincidentally, there's an example of this sort of expression in our
latest instalment of Njáls saga.

En svá fremi skalt þú rógit frammi hafa er orðin er vinátta með yðr
mikil ok þeir trúa þér eigi verr en sér.

"But you must not employ the slander till great friendship has
developed between you and they trust you no less than [they trust]
themselves."

The assumption being that everyone trusts themselves.