To a sound judgment, the most abstract truth is the most practical.Emerson
A useful metaphor for comprehending the meaning of abstract truth would beAbstract Truth is Chemical Compound.
An abstract idea is composed of both individual concepts and other abstract concepts just as a chemical compound is composed of individual atoms and molecules.
An example of a most abstract truth might be found in John Rawls book A Theory of Justice. Rawls concludes that an apt metaphor might be Justice is Fairness.
I guess that every child develops many concepts of fairness as s/he grows up. Mom, Chris is cheating. Mom, Chris always takes the bigger piece of cake. Pick on someone your own size. The abstract concept fairness would contain many other abstract concepts and individual literal concepts. There would also be additional abstract concepts making up the abstract metaphor Justice is Fairness.
Of course one must examine the meaning of a sound judgment. I would say that Critical Thinking skills are a necessary but not sufficient component of sound judgments. Since everybody considers themselves to be a critical thinker I would have to define several different levels of critical thinking.
A level one critical thinker is a trust but verify critical thinker. A level two critical thinker is one who has taken the college course Logic 101 or has studied this subject matter on their own. The level three critical thinkers I call CT (Critical Thinker). This is a student of rational thinking who has mastered Logic 101 plus the character traits and attitude of sound critical thinking.
Can you think of an abstract idea that might be more practical than justice is fairness?