Paul Dirac - The Principles of Quantum Mechanics (1930)

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Mathematics is the tool specially suited for dealing with abstract concepts of any kind and there is no limit to its power in this field.

Paul Dirac
Causality applies only to a system which is left undisturbed. If a system is small, we cannot observe it without producing a serious disturbance and hence we cannot expect to find any causal connexion between the results of our observations. Causality will still be assumed to apply to undisturbed systems and the equations which will be set up to describe an undisturbed system will be differential equations expressing a causal connexion between conditions at one time and conditions at a later time. These equations will be in close correspondence with the equations of classical mechanics, but they will be connected only indirectly with the results of observations.

Paul Dirac
The only object of theoretical physics is to calculate results that can be compared with experiment...it is quite unnecessary that any satisfactory description of the whole course of the phenomena should be given.

Paul Dirac
It has become increasingly evident in recent times, however, that nature works on a different plan. Her fundamental laws do not govern the world as it appears in our mental picture in any very direct way, but instead they control a substratum of which we cannot form a mental picture without introducing irrelevancies.

Paul Dirac
A book on the new physics, if not purely descriptive of experimental work, must essentially be mathematical.

Paul Dirac
Only questions about the results of experiments have a real significance and it is only such questions that theoretical physics has to consider.

Paul Dirac
It seems that some essentially new physical ideas are here needed.

Paul Dirac