  This article is concerned with the role of fundamental principles in
theoretical physics, especially quantum theory. The fundamental principles of
relativity will be be addressed as well in view of their role in quantum
electrodynamics and quantum field theory, specifically Dirac's work, which, in
particular Dirac's derivation of his relativistic equation for the electron
from the principles of relativity and quantum theory, is the main focus of this
article. I shall, however, also consider Heisenberg's derivation of quantum
mechanics, which inspired Dirac. I argue that Heisenberg's and Dirac's work
alike was guided by their adherence to and confidence in the fundamental
principles of quantum theory. The final section of the article discusses the
recent work by G. M. D' Ariano and his coworkers on the principles of quantum
information theory, which extends quantum theory and its principles in a new
direction. This extension enabled them to offer a new derivation of Dirac's
equation from these principles alone, without using the principles of
relativity.
