# Relational Quantum Mechanics and Probability. (arXiv:1803.02644v1 [quant-ph])

We present a discussion on the three postulates of Relational Quantum

Mechanics (RQM) and the definition of probability within this framework. The

first two RQM postulates are based on the information that can be extracted

from interaction of different systems, and the third postulate defines the

properties of the probability function. Here we demonstrate that from a

rigorous definition of the conditional probability for the possible outcomes of

different measurements, the third postulate is unnecessary and the Born's rule

naturally emerges from the first two postulates by applying the Gleason's

theorem. We demonstrate in addition that the probability function is uniquely

defined for classical and quantum phenomena. The presence or not of

interference terms is demonstrated to be related to the precise formulation of

the conditional probability where distributive property on its arguments cannot

be taken for granted. In the particular case of Young's slits experiment, the

two possible argument formulations correspond to the possibility or not to

determine the particle passage through a particular path.