sentences:  For example, if we have a probability distribution for where a particle is, it is described by the "state"
  Where formula_30 is the probability density function, a positive number that measures the probability that the particle will be found at a certain location.The evolution equation is also linear in probability, for fundamental reasons.If the particle has some probability for going from position "x" to "y", and from "z" to "y", the probability of going to "y" starting from a state which is half-"x" and half-"z" is a half-and-half mixture of the probability of going to "y" from each of the options.