Three Basic Principles of Quantum Mechanics

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

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