More Info on LSAT Logical Reasoning

This week, I want to talk about soundness. So that's what I'm going to do. YOU'RE NOT THE BOSS OF ME, WORLD.

K. Got that out of my system. Let's do this:

Soundness, as a principle, is the simultaneous measure of two distinct aspects of a deductive argument: its coherence and its validity.  OK.  So what’re coherence and validity?  We’re glad you asked!

Remember our tow-chain from two weeks ago?  Let’s get that image back in our minds to continue.  Got it?  Excellent.

Coherence is a measure of an argument’s logical connectivity; coherence measures whether or not all the pieces link together.  If all the links in our chain of evidence lead – without any gaps – to a conclusion, then our chain is continuous; it is coherent.  If you remove a link from the chain, the argument ceases to be connected; if any link is missing, the argument is incoherent.

Clearly, however, continuity – coherence – cannot be the sole measure of an argument’s (or of a tow-chain’s) utility.  You know those paper chains we used to make in third grade to decorate things like classrooms and Christmas trees?  You could have one of those that was a solid mile of uninterrupted – coherent – links, and it still wouldn’t be worth anything as a tow-chain.  It might cohere, but it wouldn’t be strong enough to pull any weight.

Validity is a measure of an argument’s truth value.  If all of the links in our chain of evidence are strong – if every piece of evidence is true – then our premises are valid.  

In the world, there are only two ways by which a deductive argument can fail:  

1. An argument is flawed if any necessary evidence offered in its support is untrue.
2. An argument is flawed if it omits any necessary evidence.

On the LSAT, there is the consistent stipulation of truth; we will accept as true anything the test writers offer into evidence.  This idiosyncrasy of the test means that for the LSAT, we can dispense with the possibility of an argument failing by unsoundness.  On the LSAT, there is only one way an argument can be flawed. 

On the LSAT, an argument is flawed only if it omits any necessary evidence.

Next week, we'll take a look at a common ommission on the LSAT. It will be AWESOME.

Questions? Comments? Complaints? Post them below, or shoot me an email.

Be good to one another, for we need it now more than ever,

d

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