system
Given the following post, respond as if you're commenting on Reddit:
human
What is this economic law called?

I'm looking for the name of the law that states that when an actor needs to make several consecutive decisions (e.g. spending another euro, working another hours) he/she should spend that on the choice that gives the most utility.

(My situation applies specifically to an organisation that aims to improve a public good, if that makes any difference.)
gpt
Not sure I'dd call it an "economic law," but you could call it the principle of utility maximization.  

There's a certain circularity to maximizing "utility," though -- to say that option A gives more utility than option B means nothing more or less than that the actor in question prefers A.  To "maximize utility" is another way of saying that an actor has consistent preferences and acts on them.

So all your "law" is saying is that you should try to take your best option.  Or, relatedly, spend each unit of your resources the best way you can.  Which is true and important, but not at all surprising -- that might be why it is not usually referred to as a "law."  Don't feel bad, though -- if you ask me, this is the fundamental principle of economics.  A lot of other stuff can be derived from it.