Three Basic Principles of Quantum Mechanics

1. The event probability P in an ideal experiment is given by the square of the absolute value of complex number which is called the probability amplitude (or shortly "Amplitude") :
P = ½f½2 = φ · φ*
2. If there are few possible events then can be applied so called superposition principle (linear combination):
f = φ1 + φ2
P = ½f½2 = ½f1 + φ2½2 ← Interference
3. If one carry out an experiment in which it is possible to choose alternative decision then the total probability is as follows:
   P = P1 + P2 = ½f1½2 + ½f2½2 ← no Interference

Contrary to classical description of nature we can only talk about the event probabilities in quantum mechanics.