top of page
  • _

How LSAT formal logic applies directly to the law

Updated: Nov 4, 2023

Please note that this article discusses formal logic concepts, such as "conditional statements" and "the contra-positive". A full explanation for these concepts won't be provided here, but that shouldn't be necessary. For anyone not familiar with these concepts, just know that this article isn't expressing an opinion about the Bill of Rights (see below), but a formulaic application of formal logic.

The LSAT Analytical Reasoning (games) section is riddled with formal logic. That is, "if-then" statements and the infamous contra-positive, the only deduction that can made from such conditional statements. We all know that the law is all about logic, which is the foundation of the LSAT, but isn't most of life all about logic? What is it about the law that requires students to agonize over switching and negating, changing "or" to "and", while remembering that "neither...nor" means "not...and"? A great answer can be found in the good old Bill of Rights.

Let's start with the Fourth Amendment and how formal logic shows that warrantless searches are actually constitutional (BTW - I'm a libertarian):

"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."

This sentence can be accurately (though not completely) rephrased into four conditional statements:

IF unreasonable search THEN search not permitted.

IF search permitted THEN search is reasonable (contra-positive of previous statement)

IF warrant THEN probable cause AND particular description.

IF no probable cause OR no particular description, THEN no warrant (contra-positive of previous).

Notice how the second conditional statement provides the only necessary condition to conduct a search: it needs to be reasonable. Nothing about warrants.

Yes, yes, this begs the question of what is meant by "reasonable", but that's beyond the scope of this article (but not my LSAT training, BTW).

For those familiar with LSAT formal logic - did you notice the "not...unless" phrase in "no Warrants...but upon"? And that's not the only place you'll see it in the Bill of Rights, but I'll get back to that.

Back to the Fourth Amendment. Nowhere does it say, or even imply, the following:

IF search permitted THEN warrant.

Or the contra-positive:

IF no warrant THEN no search permitted.

To repeat: the two above conditional statements are NOT logical deductions that can be drawn from the Fourth Amendment. The logical implications of a warrant support this:

IF warrant THEN search permitted.

Or the contra-positive

IF search not permitted THEN no warrant.

This creates an interesting connection to the explicit text, namely:

IF warrant THEN search is reasonable.

(IF warrant THEN search; IF search THEN reasonable; therefore: IF warrant THEN search is reasonable).

Or the contrapositive:

IF search is not reasonable, THEN no warrant.

Fun, right? Head spinning yet? If not, then congratulations - you're ready for the LSAT. If so, then rest assured that many students are in the same boat. Just know that formal logic isn't just some abstract concept designed to drive students crazy. In fact, it has actual life or death implications, as found in parts of the Fifth Amendment:

"No person deprived of life, liberty, or property, without due process of law..."

Accurately rephrased into a conditional statement:

IF deprivation of life THEN due process.

Or the contra-positive:

IF no due process THEN no deprivation of life.

Notice the "not...unless" statement ("no...without")?


15 views0 comments

Recent Posts

See All

LSAT Study Tips

The following should be viewed as a supplement to and NOT substitute for in-depth LSAT prep. Book recommendations (in alphabet order): Loophole, LSAT Trainer, Princeton Review, PowerScore. Course reco

Achieving the Right Mindset for Test Day Success

The following is based on my 20+ years of experience working with LSAT and GMAT students and my long interest in neuroscience. I suppose the fact that my parents are retired psychotherapists doesn’t h


bottom of page