Only probabilistic statements about the results of a measurement on a physical system can be made. The probability that a system be in the state | ¡ > when it is known to be in the state ] d) > is 1< x l,þ >l' (2.1) Chapter 5 contains a geoeral description ¿¡d discussio¡ of mixed states. QLB: Scanerinï Theory Chapter 3 coDtai¡s a descriptioú a¡d discussio¡ of mixed states iD scattering theory. The complex number' < X I ú > is the probabiliQ amplifude that the system be in the state l¡ > when it is known to be in the state l/ >. 3. If | ór >,1 ó, >.,-.. ,l 4,,, > a¡e all the possible outcomes of a measurement of the state of a system, then fo¡ any states I X > ^d l rþ >, *a < x I tþ >: L. * I ót, >< fu l,þ > À:l (2.2) (2.2) is the Principle of Superposition.