“We are going to the moon that is not very far. Man has so much farther to go within himself.”

Mechanics of the Mind (1977, Cambridge University Press).

1961, Speech to Special Joint Session of Congress
Context: I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish. We propose to accelerate the development of the appropriate lunar space craft. We propose to develop alternate liquid and solid fuel boosters, much larger than any now being developed, until certain which is superior. We propose additional funds for other engine development and for unmanned explorations — explorations which are particularly important for one purpose which this nation will never overlook: the survival of the man who first makes this daring flight. But in a very real sense, it will not be one man going to the moon — if we make this judgment affirmatively, it will be an entire nation. For all of us must work to put him there.

Book 1, p. 8
Cosmotheoros (1695; publ. 1698)

Source: Ages in Chaos (2003), Chapter 15, “The world was tired out with geological theories” (p. 160)

As quoted by Bryant, ibid.
Astronomia nova (1609)

The Tragic Sense of Life (1913), VIII : From God to God

Source: What the Bones Tell Us (1997), Ch. 1

Life Without Principle (1863)

Variant: The farther you go, however, the harder it is to return. The world has many edges, and it's easy to fall off.
Source: Dispatches from the Edge: A Memoir of War, Disasters, and Survival