There may not be much more important to economics than marginal utility.  Acting man will value things for the removal of uneasiness.  They are valued by what is less or more urgent.  Man will have a scale of values.  Each unit of a good will go to satisfying his most urgent desire first.  The actor is interested in the units of that thing.  If they are not interchangeable units, equally capable of rendering the same service, it’s not the same unit.

If a unit of a good is available, it will go to satisfying his most urgent desire first.  As a second unit is available, it’s less valuable to the actor than the first.  The unit will be lower on his scale of values.  Each unit added to the stock is less valuable than the previous.  Thus, each unit added has less utility than the previous.  Your bank account has a utility at this moment in time.  Each dollar will go to removing the uneasiness highest on your value scale.

Your first dollar may go to satisfying your basic needs first.  You’ll make sure you have a place to live, and all other basic needs are met.  The additional money will go to the next most urgent desire.  This may be improving the quality of food.  Food doesn’t belong to a vague class of nourishment, but in marginal units of an interchangeable good.  Remember, you must choose between marginal units.  Don’t get stuck in the value paradox.

It is important to remember values are subjective.  The actor will satisfy his most urgent desire first, but they can’t be measured.  Likewise, interpersonal comparisons can’t be made.  If they can’t be measured, interpersonal comparisons can’t be made, they can’t be placed into a formula, model, or anything else.  There is no arithmetical process.  If none of this can be done, how do they justify redistribution?

They ignore marginal utility.  The rich man has more money than the poor man.  They make interpersonal comparisons, a critical intentional error.  They assume every dollar has the same utility.  The poor man will gain more utility than the rich man loses, and social utility increases.  However, value is subjective.  The rich man may utilize the money better.  So, this would justify redistribution from the poor to the rich.  The logic for redistribution is extremely flawed.

References

Ludwig von Mises; Human Action

Murray Rothbard; Man, Economy, and State