Hello! I am reading some introductory stuff on Klein-Gordon equation and I see that the author mentions sometimes that in a certain context the K-G equation "is a classical field equation, not a quantum mechanical field equation". I am not sure I understand. What is the difference between the two? Isn't the wave function a field, as it assigns to each point in space a number? I.e. can someone give me a clear explanation of what is the difference between a classical field, the wave function of a particle and a quantum field? Thank you!