He knew but one secret, which was to do one thing at a time

From Remaking the World by Henry Petroski, page 68.

It is not only practicing engineers who seem to have learned the importance of keeping their minds under one hat while thinking about a hard problem. In his Journal of 1844, Ralph Waldo Emerson wrote under the heading "Otherness":

Henry Thoreau said, he knew but one secret, which was to do one thing at a time, and though he has his evenings for study, if he was in the day inventing machines for sawing his plumbago, he invents wheels all the evening and night also; and if this week he has some goo reading and thoughts before him, his brain runs on that all day, whilst pencils pass through his hands.

While everyone, engineer and nonengineer alike, has experienced the feeling of being completely absorbed by whatever the mind is deeply engaged in at any given moment, it may be especially reassuring that so many engineers seem to have spent so many sleepless nights while their designs were progressing from the back of an envelope through increasingly complex and detailed calculations and drawings to the realization in an artifact upon whose safety the lives of so many depend.