To Enjoy the Morning Flower in the Evening -- Where is the Subtlety of   Quantum Mechanics?
  Why does the $i=\sqrt{-1}$ appear essentially in the quantum mechanics? Why are there operators and noncommutativity (the uncertainty relation) in the quantum mechanics? Why are these two aspects closely related and indivisible? In probing these problems, a new point of view is proposed tentatively. 
Guang-jiong Ni (Department of Physics, Fudan University and Shanghai,   P. R. China)