“The ideal and the real are not mutually exclusive. A thing may be ideal and also real.”

Source: Old Man's War

Source: Halakhic Man (1983), p. 19

Upon the fall of his ministry; said to journalist Sir Henry William Lucy, The Diary of a Journalist (Vol. 1), E. P. Dutton, 1920), p 93.

Too Far To Go, foreword (1979)

Prose Papers on Poetry Macmillan & Co 1910.
Prose Papers on Poetry (1910)

Source: The State — Its Historic Role (1897), I
Context: The Roman Empire was a State in the real sense of the word. To this day it remains the legist's ideal. Its organs covered a vast domain with a tight network. Everything gravitated towards Rome: economic and military life, wealth, education, nay, even religion. From Rome came the laws, the magistrates, the legions to defend the territory, the prefects and the gods, The whole life of the Empire went back to the Senate — later to the Caesar, the all powerful, omniscient, god of the Empire. Every province, every district had its Capitol in miniature, its small portion of Roman sovereignty to govern every aspect of daily life. A single law, that imposed by Rome, dominated that Empire which did not represent a confederation of fellow citizens but was simply a herd of subjects.
Even now, the legist and the authoritarian still admire the unity of that Empire, the unitarian spirit of its laws and, as they put it, the beauty and harmony of that organization.
But the disintegration from within, hastened by the barbarian invasion; the extinction of local life, which could no longer resist the attacks from outside on the one hand nor the canker spreading from the centre on the other; the domination by the rich who had appropriated the land to themselves and the misery of those who cultivated it — all these causes reduced the Empire to a shambles, and on these ruins a new civilization developed which is now ours.

As quoted in "The Mystery Of Marie Rogêt" (1842) by Edgar Allan Poe, adapted from Fragments from German Prose Writers (1841) by Sarah Austin
Context: There are ideal series of events which run parallel with the real ones. They rarely coincide. Men and circumstances generally modify the ideal train of events, so that it seems imperfect, and its consequences are equally imperfect. Thus with the Reformation; instead of Protestantism came Lutheranism.

The Evolution of Physics (1938) (co-written with Leopold Infeld) <!-- later published by Simon & Schuster (1967) -->
1930s
Context: Physical concepts are free creations of the human mind, and are not, however it may seem, uniquely determined by the external world. In our endeavor to understand reality we are somewhat like a man trying to understand the mechanism of a closed watch. He sees the face and the moving hands, even hears its ticking, but he has no way of opening the case. If he is ingenious he may form some picture of a mechanism which could be responsible for all the things he observes, but he may never be quite sure his picture is the only one which could explain his observations. He will never be able to compare his picture with the real mechanism and he cannot even imagine the possibility or the meaning of such a comparison. But he certainly believes that, as his knowledge increases, his picture of reality will become simpler and simpler and will explain a wider and wider range of his sensuous impressions. He may also believe in the existence of the ideal limit of knowledge and that it is approached by the human mind. He may call this ideal limit the objective truth.