As Oliver accompanied his master in most of his adult expeditions too, in order that he might acquire that equanimity of demeanour and full command of nerve which was essential to a finished undertaker, he had many opportunities of observing the beautiful resignation and fortitude with which some strong-minded people bear their trials and losses.
There are no more uses of "essential" in the book.
Show samples from other sources
The essential feature of the pen is that it writes so consistently.
I carry all the essential tools and materials I need in this toolbox.