“Difficulties increase the nearer we get to the goal.”

A Vindication of Natural Society (1756)
Context: You are, my Lord, but just entering into the world; I am going out of it. I have played long enough to be heartily tired of the drama. Whether I have acted my part in it well or ill, posterity will judge with more candour than I, or than the present age, with our present passions, can possibly pretend to. For my part, I quit it without a sigh, and submit to the sovereign order without murmuring. The nearer we approach to the goal of life, the better we begin to understand the true value of our existence, and the real weight of our opinions. We set out much in love with both; but we leave much behind us as we advance. We first throw away the tales along with the rattles of our nurses; those of the priest keep their hold a little longer; those of our governors the longest of all. But the passions which prop these opinions are withdrawn one after another; and the cool light of reason, at the setting of our life, shows us what a false splendour played upon these objects during our more sanguine seasons. Happy, my Lord, if, instructed by my experience, and even by my errors, you come early to make such an estimate of things, as may give freedom and ease to your life. I am happy that such an estimate promises me comfort at my death.

Source: The Tragedy of Great Power Politics (2001), Chapter 1, Introduction, p. 20

Letter to Moritz von Egidy (c. January 1894) - Original German text online http://www.lilienthal-museum.de/olma/l1852.htm
Context: I, too, have made it a lifelong task of mine to add a cultural element to my work, which should result in uniting countries and reconciling their people. Our experience of today's civilisation suffers from the fact that it only happens on the surface of the earth. We have invented barricades between our countries, custom regulations and constraints and complicated traffic laws and these are only possible because we are not in control of the 'kingdom of the air', and not as 'free as a bird'.
Numerous technicians in every state are doing their utmost to achieve the dream of free, unlimited flight and it is precisely here where changes can be made which would have a radical effect on our whole way of life. The borders between countries would lose their significance, because they could not be closed off from each other. Linguistic differences would disappear, as human mobility increased. National defence would cease to devour the best resources of nations as it would become impossible in itself. And the necessity of resolving disagreements among nations in some other way than by bloody battles would, in its turn, lead us to eternal peace.
We are getting closer to this goal. When we will reach it, I do not know.

On War (1832), Book 2

Source: The Wealth of Nations (1776), Book I, Chapter IX, p. 111.

Source: Time Travelers Never Die (2009), Chapter 29 (p. 270)

"Remaining Awake Through a Great Revolution" (31 March 1968)
1960s
Context: I say to you that our goal is freedom, and I believe we are going to get there because however much she strays away from it, the goal of America is freedom. Abused and scorned though we may be as a people, our destiny is tied up in the destiny of America.

as quoted by Hendrik Casimir in an interview http://www.aip.org/history/ohilist/4550_1.html by Thomas Kuhn, Léon Rosenfeld, Aage Bohr and Erik Ruedinger at the Institute for Theoretical Physics, Copenhagen, 5 and 6 July, 1963.
Attributed