If you want to succeed at the LSAT, you really should get to know prescriptive statements. That’s a prescriptive statement — a claim about what “should” or “ought to” be done. These kinds of claims come up a lot on the LSAT and it’s helpful to know what they mean. They also give you hints that certain other things might be going on. Let’s talk a bit about prescriptive statements.
Prescriptive statements are often given as an argument’s conclusion. They have a high burden of proof — it’s hard to prove what should be done without premises about what should be done. Take the following argument, for example:
Drinking to excess gives you a bad headache. When you have a bad headache you can’t get your work done. So you shouldn’t drink to excess.
The argument’s premises identify bad things about drinking to excess. The conclusion is that you shouldn’t do it. Seems reasonable. But hold on. Could there be good things, too? The argument requires the assumption that the bad things in the premises are enough to mean that you shouldn’t do it.
Arguments like the above often show up in questions that ask for a principle that would strengthen the argument. Most often, the answer’s job is to supply the missing premise about what should or shouldn’t be done. In this case, the correct answer might be something like, “You shouldn’t do things that interfere with getting your work done.” That answer would help link the factual premises to the prescriptive conclusion. This argument could come up in other types of questions, too, and the answer might relate to whether or not there are good things about drinking to excess and whether those good things might outweigh the bad.
In other contexts, a prescriptive conclusion can be a hint that there’s some causal reasoning going on. Check out the following argument:
People who live in Bel Air are much wealthier than average. So if you want to make a lot of money, you should move to Bel Air.
The assumption is that people are rich because they live in Bel Air, when it might well be that the converse is true — that they live in Bel Air because they’re rich. In general, this kind of argument shows up a lot in Strengthen and Weaken questions. The premises identify a correlation, and the conclusion makes a prescriptive recommendation, often qualified with “if you want to” or something like that. That’s pretty much always a causal argument and you’d want to strengthen or weaken the argument by addressing the underlying causal relationship.
In general, the word “should” should make your ears perk up. It’s hard to prove what you should do. Watch out for incomplete information in the premises and bad causal assumptions. Keep in mind that even if the argument seems pretty reasonable, you’ll still need a premise about what should be done to make a prescriptive conclusion completely valid.