a linear theory (quantum theory)?

[xylai]

Some textbooks say that quantum mechanics is a linear theory. I don't know why?
How can we decide a theory is a linear theory?
Thank you!

[Doc Al]

The governing equation (the Schrodinger Equation) is a linear differential equation. One consequence of linearity is that if \psi_1 and \psi_2 are both solutions, then so is \psi_1 + \psi_2.

[xylai]

Thank you!

[Marco_84]

http://en.wikipedia.org/wiki/Superposition_principle