Answer: passion is supposed to refer itself to a psychic disposition, a movement of soul or feeling, while reason is allegedly a facculty of understanding. Definitions on the latter are though extremely weak even in philosophy, and little can be concluded from etymology: actually from latin ‘ratio’ = measure, part, from where the Spanish or French ‘ration’ or ‘ración’. The Greek equivalent, better defined, is ‘nous’ a human facculty going beyond understanding and seeming to be the transformation of instinctive intuition into a spiritual perception. The difficulty of the Roman to understand this notion allows thinking a transformation leaning more on ‘divisions of understanding’ than on intuition, as if the balance obtained through immediate apprehension could be transformed into figures.
To give an example: a Greek would measure the part of food through the immediate need of the person, which may depend on his hunger or on the difficulty of his task. A Roman would give to each the same portion, independently of other factors. The difficulty of the Greek notion is that it has to apprehend the subjective wanting as possibly misleading: to claim for something does not necessarily mean that you need it, and to put a frein to claims of the kind ‘but he has more than me’ seems far too difficult to the Roman, so that he solves the Alexandrian knot with a simple sword cut.
In time (around the XVIth century), this ‘nous’ would become the ‘faculty of judgement’ in philosophy, while ‘reason’ will become some kind of ‘absolute referential’ which may be ideal (Kant) or subjective, and than associable to what is called common reason (Rousseau).
In any case, reason as possible means to control passion (Plato), which is often attached to the irrational, absurd, disordered, has largely been favoured all over history in philosophical currents, while reality does seem to give largest spaces of expression to … passion.