The 1935 debate opposing Einstein, Podolsky and Rosen to Bohr elicited so many comments and developments, both theoretical and experimental, until this day, that the main point at stake at that time can be overlooked by modern readers, especially students. This paper draws the reader's attention to the historical background of Einstein's paper and Bohr's reply. We show that Einstein's definition of a complete physical theory is taken from Mach's criticism of atomic theory based upon classical-mechanical views. As for Bohr's definition of physical reality, it can be simply understood by reference to classical physics although it was embedded in the quantum-mechanical formalism.