“In physics we have dealt hitherto only with periodic crystals.”

In 'The Problem of Increasing Human Energy: With Special Reference to the Harnessing of the Sun’s Energy', Century Illustrated Magazine (Jun 1900), 60, No. 2, 180.

Quotes, DNC Address (2004)
Context: We have to be crystal clear about the threat we face from terrorism. It is deadly. It is real. It is imminent. But in order to protect our people, shouldn't we focus on the real source of this threat: the group that attacked us and is trying to attack us again: Al Qaeda, headed by Osama bin Laden? Wouldn't we be safer with a President who didn't insist on confusing Al Qaeda with Iraq? Doesn't that divert too much of our attention away from the principal danger?

Variant: A bookstore is one of the many pieces of evidence we have that people are still thinking.

The Development of Quantum Mechanics (1933)
Context: However the development proceeds in detail, the path so far traced by the quantum theory indicates that an understanding of those still unclarified features of atomic physics can only be acquired by foregoing visualization and objectification to an extent greater than that customary hitherto. We have probably no reason to regret this, because the thought of the great epistemological difficulties with which the visual atom concept of earlier physics had to contend gives us the hope that the abstracter atomic physics developing at present will one day fit more harmoniously into the great edifice of Science.

The external world of physics has thus become a world of shadows. In removing our illusions we have removed the substance, for indeed we have seen that substance is one of the greatest of our illusions. Later perhaps we may inquire whether in our zeal to cut out all that is unreal we may not have used the knife too ruthlessly. Perhaps, indeed, reality is a child which cannot survive without its nurse illusion. But if so, that is of little concern to the scientist, who has good and sufficient reasons for pursuing his investigations in the world of shadows and is content to leave to the philosopher the determination of its exact status in regard to reality. In the world of physics we watch a shadowgraph performance of the drama of familiar life. The shadow of my elbow rests on the shadow table as the shadow ink flows over the shadow paper. It is all symbolic, and as a symbol the physicist leaves it. Then comes the alchemist Mind who transmutes the symbols. The sparsely spread nuclei of electric force become a tangible solid; their restless agitation becomes the warmth of summer; the octave of aethereal vibrations becomes a gorgeous rainbow. Nor does the alchemy stop here. In the transmuted world new significances arise which are scarcely to be traced in the world of symbols; so that it becomes a world of beauty and purpose — and, alas, suffering and evil.
The frank realisation that physical science is concerned with a world of shadows is one of the most significant of recent advances.

Introduction
The Nature of the Physical World (1928)

Source: Looking Backward, 2000-1887 http://www.gutenberg.org/dirs/etext96/lkbak10.txt (1888), Ch. 6.

Source: The Last Continent

Science and the Unseen World (1929)
Context: It remains a real world if there is a background to the symbols—an unknown quantity which the mathematical symbol x stands for. We think we are not wholly cut off from this background. It is to this background that our own personality and consciousness belong, and those spiritual aspects of our nature not to be described by any symbolism... to which mathematical physics has hitherto restricted itself.<!--III, p.37-38

Session 669, p. 387
The Nature of Personal Reality (1974)