The best mental models are ones that highlight and clarify those aspects of reality that are most likely to enable us to muster appropriate means for the achievement of our ends.

One important aspect of reality that sound economics highlights and clarifies is the unavoidability and ubiquity of scarcity.  "The economic problem" (as it is commonly called) exists only because not all human wants can be satisfied with the means – resources, tools, time, knowledge  – available to us.  Some – indeed, most – of our desires will forever go unsatisfied in order that we might use the scarce resources, tools, time, and knowledge that we have to satisfy those relatively few desires that we judge to be the most important.

We – as individuals and as society – never act perfectly.  We make lots of mistakes.  But if we keep our mistakes to some achievable minimum, we'll prosper.  So we must beware – again, both as individuals and as society – of our proneness to error.  The successful amongst us learn from our mistakes; those amongst us who do not learn from mistakes fail.