The extremely short description of legal or scholastic reasoning is to think of a proposition or dispute as Schrodinger's Cat, both true and false at the same time, or each party at fault or not at the same time, or the appropriate dichotomy. Then gather all the moral or legal disputes that are similar to this one. Argue by analogy for each side both from the facts of those prior disputes and from the informal rules ("holdings") implied by the decisions resolving those disputes. This kind of reasoning allows a lawyer to anticipate an opponent's as well as their own argument in a case, and allows a judge to appreciate both sides of an argument, the latter also crucial, but often absent, in reasoning in about politics, morals, and the more complex areas of science, which in absence of this kind of discipline is dominated by confirmation bias and lack of understanding of other points of view.

This is a moderately reasonable model of litigation, but it isn't complete. For example, Thurgood Marshall litigated separate-but-equal in the law school context specifically because every judge has a gut feeling of how to compare law schools, which just isn't true about other educational institutions. In law school, I heard the apocryphal story that the law for the State of Texas argued that the new segregated law school was just as good as UT Law School, and Justice Clark - a graduate of UT - passed a note to a colleague that read "Bullshit" That's clever lawyering and has nothing to do with arguing from precedent.

Further, not all law is litigation. The legislature empowered to make new laws that have no relationship to old laws. In short, there's a fair amount more to the practice of law than reasoning by analogy, even if reasoning by analogy is an important skill for a lawyer.