"Not until" in conditional reasoning

[fg6118]

For a sentence that says "not until teachers have the power to make decisions in their own classrooms can they enable their students to make their own decisions” Does "not until" always introduce a necessary condition? And are "only when" and "not unless" equivalents of "not until" where it would simply introduce a necessary condition and no need for treating it like you would "unless" (where unless introduces the necessary condition and you negate the other part of the statement and make it your sufficient condition)?

Thanks for the help!

[Nikki Siclunov]

Hi f,

Thanks for the question. “Not until’ is primarily a temporal preposition, which literally means “not before a stated time or event.” So, not every sentence using that construction will convey conditional dependence. When it does, the grammatical structure of these sentences is shown in the diagram below:

• Not until [1st clause: subject + auxiliary verb] [2nd clause: auxiliary verb + subject]

The conditional relationship between the two clauses is as follows:

• S N

• 2nd clause 1st clause

For example:

Not until [the car came to a complete stop] [did we see the damage to the front bumper]

• See damage Complete stop

Not until [the stars aligned] [did I win the lottery]

• Win lottery Stars aligned

Not until [did I stop worrying about my future]

• Stop worrying Got into Yale

In all of these examples, the clause that follows immediately after the preposition "not until" is the necessary condition. The remainder of the sentence functions as a sufficient condition. The same would be true with "not unless" and "only if/when." These are all synonymous constructions.

Hope this helps!

[fg6118]

Wow, thanks for that detailed explanation. Makes much more sense now!

[fg6118]

Hi Nikki,

Wow, the blog post you created that addresses these complicated conditional reasoning indicators in more detail is so helpful http://blog.powerscore.com/lsat/beyond- ... n-the-lsat

On that note, I have a question about "all except, all but" category you discuss in the post. It is essentially establishing that 1 and only 1 of the 2 terms must be in/ occur/ take place (like a loner variable), not allowing for the possibility that both are out/negated or the possibility that both are in/ occurring/ taking place, whereas "if but only if" says both must be in or out (like an inseparable variable pair), right? Also would it be appropriate to say "all except" and "if but only if" are opposites, since they're both double arrows but one is with a single negated variable (the "all except" statements) or one is with both variables that are either positive/in or negated/out (the "if but only if" statements)? Drawing connections between the types of relationships these indicators represent helps me understand.

Thanks so much!

[Nikki Siclunov]

Hi fg6118,

You've nailed it on the head

Phrases such as all except A are B and all but A are B are exceptive propositions, and consequently yield pairs of conjoined categorial propositions. The inferences resulting from these propositions are as follows:

A B (contrapositive: B A
A B (contrapositive: B A)

Thus:

A B (contrapositive: B A)

The relationship is bi-conditional and can be represented with the same Double-Arrow that we use to diagram if and only if relationships. As far as truth value is concerned, only two outcomes are possible in each case:

All except A are B
A B
Outcome: either A or B, but not both, must occur. In other words, one is either A or B: no one is neither, and no one is both.

A if and only if B
A B
Outcome: either A and B must both occur, or else neither A nor B can occur. In other words, one is either both A and B, or else one is neither A nor B.

Once you compare these propositions side by side, you realize that they are logical opposites of each other. You came to this realization yourself, which is totally awesome


By contrast, phrases such as none except A are B are exclusive (not exceptive) propositions, synonymous with the word only.

[fg6118]

Ok, good to know. This was so helpful. Thanks again