Living is worthwhile if one can contribute in some small way to this endless chain of progress. - Paul Adrien Maurice Dirac

In the absence of experimental evidence, basic beliefs of theoretical physicists may initially have almost a religious flavor, guided by faith and aesthetics. Fortunately unlike religion, these beliefs soon face the hard test of experiment.

The successful development of science requires a proper balance to be maintained between the method of building up from observations and the method of deducing by pure reasoning from speculative assumptions.

We may define an object to be big when the disturbance accompanying our observation of it may be neglected, and small when the disturbance cannot be neglected... In order to give an absolute meaning to size, such as is required for any theory of the ultimate structure of matter, we have to assume that there is a limit to the fineness of our powers of observation and the smallness of the accompanying disturbance... If the object under observation is such that the unavoidable limiting disturbance is negligible, then the object is big in the absolute sense and we may apply classical mechanics to it. If, on the other hand, the limiting disturbance is not negligible, then the object is small in the absolute sense and we require a new theory for dealing with it.