Is that what you meant by having to appeal to "soul"?

There still doesn't need to be an appeal to anything in material in order to define worth, not anymore than one needs to appeal to something immaterial do define weight in terms of mass. Have the utility of water be proportional to the thirst of the person, where thirst is also a definable physiological state, with "worth" being a scale factor. Thus "worth" would have a uniquely defined value regardless of person or situation.

Or at least that's one way to do it.