"Sidelights on Relativity" by Albert Einstein

Kindle edition here.

• "One reason why mathematics enjoys special esteem, above all other sciences, is that its laws are absolutely certain and indisputable, while those of all other sciences are to some extent debatable and in constant danger of being overthrown by newly discovered facts."

• "Newton's theory is probably the greatest stride ever made in the effort towards the causal nexus of natural phenomena."

• "And yet this theory evoked a lively sense of discomfort among Newton's contemporaries, because it seemed to be in conflict with the principle springing from the rest of experience, that there can be reciprocal action only through contact, and not through immediate action at a distance."

• "light must be interpreted as a vibratory process in an elastic, inert medium filling up universal space"

• "this medium, the ether, must be of the nature of a solid body, because transverse waves are not possible in a fluid, but only in a solid"

• "the purely mechanical view of nature was gradually abandoned"

• "He brought theory into harmony with experience by means of a wonderful simplification of theoretical principles."

• "For the theoretician such an asymmetry in the theoretical structure, with no corresponding asymmetry in the system of experience, is intolerable."

• "both matter and radiation are but special forms of distributed energy, ponderable mass losing its isolation and appearing as a special form of energy"

• "There may be supposed to be extended physical objects to which the idea of motion cannot be applied."

• "In Minkowski's idiom this is expressed as follows:--Not every extended conformation in the four-dimensional world can be regarded as composed of world-threads."

• "To deny the ether is ultimately to assume that empty space has no physical qualities whatever."

• "what is essential is merely that besides observable objects, another thing, which is not perceptible, must be looked upon as real"

• "There can be no space nor any part of space without gravitational potentials."

• "The existence of the gravitational field is inseparably bound up with the existence of space."

• "According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."

• "One reason why mathematics enjoys special esteem, above all other sciences, is that its laws are absolutely certain and indisputable, while those of all other sciences are to some extent debatable and in constant danger of being overthrown by newly discovered facts."

• "At this point an enigma presents itself which in all ages has agitated inquiring minds. How can it be that mathematics, being after all a product of human thought which is independent of experience, is so admirably appropriate to the objects of reality? Is human reason, then, without experience, merely by taking thought, able to fathom the properties of real things."

• "As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality."

• "under closer inspection the real solid bodies in nature are not rigid" 

• "Can we picture to ourselves a three-dimensional universe which is finite, yet unbounded?"
• "Nobody can imagine this thing," he cries indignantly. "It can be said, but cannot be thought.
• "We must try to surmount this barrier in the mind"