fresh horses

It is a profoundly erroneous truism, repeated by all copy-books and by eminent people making speeches, that we should cultivate the habit of thinking of what we are doing. The precise opposite is the case. Civilization advances by extending the number of operations which we can perform without thinking about them. Operations of thought are like cavalry charges in a battle—they are strictly limited in number, they require fresh horses, and must only be made at decisive moments.

A.N. Whitehead, An introduction to mathematics, 1911 New York: Holt, p. 41

He makes this observation while discussing mathematical symbolism:

One very important property for symbolism to possess is that it should be concise, so as to be visible at one glance of the eye and to be rapidly written.