Remember that the state vector should have elements corresponding to the
probability of each of our possibilities phrases being the current phrase. Of course
this means that the sum of entries in this vector is one. (Why?)

I'm thinking of saying something like, since one of the phrases has to be said, then the probability of a phrase being said is 1. Therefore since there are several phrases with different probabilities, they all have to add up to 1. I'm pretty sure this is terribly worded, any help?