Formal Logic vs LSAT Logic

[nmmizokami]

I've been studying for the LSAT for some time now but have recently been struggling certain question types (inference, sufficient assumption, parallel reasoning. My goal is to score in the top percentiles so I am exploring every route to get there.

I've taken logic classes in college so I am somewhat familiar with formal logic. Recently, I've begun reviewing material and have found some contradictions in the way LSAT logic is diagrammed. For instance, diagramming E propositions (No S is P) as S ---> ~P and its contrapositive P ---> ~S. The contrapositive inference is intuitively apparent but according to formal logic is invalid. Moreover, existential import seems to not matter on different inference type questions.

Anyway, am I getting too deep into formal logic or is there some benefit to understanding this? Again, I am attempting to score in the very upper echelons and one area of weakness is abstracting specific argument forms. Has anyone been able to leverage this knowledge to their advantage?

Thanks in advance!

[ohthatpatrick]

Sorry for the late reply. I do not think that formal logic classes are of much help and could easily see them possibly being a hindrance (for example, your qualms about deriving the legal contrapositive of P --> ~S, which is definitely legitimate on LSAT)

The extent of conditional logic on the test is pretty minimal and consistent.

You need to know how to correctly translate a Rule, Universal, Guarantee, or Requirement into the correct "if, then" form. And you need to know how think about and/or diagram the contrapositive (including knowing that 'and' and 'or' switch in the contrapositive).

It's worth memorizing the common words that indicate LEFT SIDE (sufficient) conditions:
if, when, whenever, all, each, any, every, no, the only

It's worth memorizing the common words that indicate RIGHT SIDE (necessary) conditions:
only, only if, will, implies, ensures, leads to, requires, must, necessitates, presupposes, depends on

Memorize the terms that mean "if not":
unless, without, until

And memorize the terms that give you either/or's (i.e. "biconditionals"):
if and only if, then and only then, when and only when

That covers 98% of cases.

Hope this helps.