# Analogy of Set and Subset and Contracts in abstracto and Marriage in concreto/in particular

I had a talk with a professor of family law and we are frequently told that there are general ordinances for contracts in general and particular ordinances for marriage.

I am problematised by the logic. The general ordinances are only in vigour under the condition that there is no contrasting lex specialis, a contrasting particular ordinance.

So I thought that would lift the generality of the general provisions. They are particular in that they do not hold for all contracts but only for any contracts that are not marriage.

But I was told that the analogy to Euclidean geometry (everything that holds for rhombi necessarily also holds for squares) is inapplicable. Nonetheless, logic and set theory still hold in law as stated by the Professor.

My question: Does logic and set theory allow for properties holding for all members of a set (something which I identify with generality) not to hold for a particular subset?

• There are all kinds of properties that you can define on an element relative to the set it is a member of. Like set X = {1,2} Y ={1} Z={2}, define property `alone` for any element that if it is removed the result would be an empty set. Notice how it's true for bot Y and Z but not for X. Feb 23, 2020 at 22:34
• No, they do not allow such properties and neither does law. In legal reasoning "general" is simply used differently. It does not mean general in the usual sense, it means "holds unless specifically overruled". Feb 24, 2020 at 4:40
• In ordinary language we choose to use "straight" loosely, e.g. rulers are "straight". It doesn't mean that anything happens to the properties themselves, only to how we talk about them. "General" legal properties aren't general, strictly speaking, just as rulers aren't straight. Feb 24, 2020 at 6:56
• @GeorgeNtoulos if you do not allow exceptions, there are no exceptions. That's logic. As Conifold said it, real life is messier than that. I was trying to say the same thing but differently. You misunderstand that properties that are considering externalities are not 'in' the elements, but it's not important. The point is that you seem to be expecting a level of uniformity from general practice that is impossible to adhere to and which no one is actually trying to achieve, hence the confusion. Feb 25, 2020 at 8:09
• "If general suddenly became "holds for all members unless specifically overruled" then we do not have any problems in general properties not to hold for a subset". Sure, if "general" is meant in the legal sense (it isn't "sudden", general-strictly-speaking is simply not useful in legal practice). But then "general" does not mean "shared by all elements of the parent set" anymore. Just as if we agree to call crooked "straight" there is no problem with calling the hunchback of Notre Dame "straight". But it doesn't change anything about his properties. Feb 25, 2020 at 12:27