> the more of something you have, the less of it you want. This phenomenon is referred to as diminishing marginal utility by economists.
Okay, but I have a counter-example. Money. The people who have the most sometimes still act like penny-pinchers, and lust after even more, and haggle over the smallest things. Therefore the "law of diminishing marginal utility" is more a rule of thumb than a law.
If you own a good G, that you could use to satisfy either a desire D1, or a competing desire D2, you will logically choose to use G to satisfy the greater one of these two desires - the one with the greater subjective utility to you. Let's assume that is D1.
Now, if instead of 1 you have 2 of G, you can satisfy both D1 and D2. As we have already established that the utility you have derived from consuming the first G to achieve D1 must be greater than the utility derived from consuming the second G to achieve D2, the utility of the additional unit of G is diminished when compared to the previous one.
https://oll.libertyfund.org/title/greaves-human-action-a-tre...
https://mises.org/library/man-economy-and-state-power-and-ma...
Anyway, if it isn't -- no, there are no universal economic laws, for the very simple reason that "humans are not always rational" and economics is fundamentally about human behavior, in groups, no less.
Let's do us all one better and nail this down.
"Predictable, repeatable action"
Repeatable is even untrue, unless I misunderstand your meaning. First of all, the ranking of your goals or desires is not assumed to be constant over time. Also, it is not assumed that your world model is constant. Quite the opposite, it is often explicitly emphasized that humans have the ability to change their mind about things, for example by learning.
I've got great news for you! There are universal economic laws that always hold true in all situations.
For example, "man acts". You can't deny that, because denying it would be an act in itself, leading to a (performative) contradiction. From this law, and just a few others of a similar kind, you can build a whole structure of logically provable theorems that have profound implications for economics and the social sciences in general.