## Deduction Question

Prepare for the LSAT or discuss it with others in this forum.
signature17

Posts: 4
Joined: Tue Nov 10, 2009 4:20 pm

### Deduction Question

I'm having trouble understanding this idea of deduction.

Group Game Example: Group X will lobby for Animal A if, and only if, Group Y does so as well.

I understand the forward relationship, but apparently "if, and only if" means that Group Y can not lobby for Animal A unless Group X does as well.

Can somebody explain how this is determined?

Richie Tenenbaum

Posts: 2118
Joined: Wed Dec 31, 2008 6:17 am

### Re: Deduction Question

signature17 wrote:I'm having trouble understanding this idea of deduction.

Group Game Example: Group X will lobby for Animal A if, and only if, Group Y does so as well.

I understand the forward relationship, but apparently "if, and only if" means that Group Y can not lobby for Animal A unless Group X does as well.

Can somebody explain how this is determined?

"if, and only if," translates into a double arrow (<--->) since it is actually 2 different formal logic statements

1) X lobby for A if Group Y (aka if group Y then X lobby for A)
2) X lobby for A only if (aka then) Group Y

Thus we know, X lobby for A <---> Group Y

bedefan

Posts: 172
Joined: Sat Apr 24, 2010 10:39 am

### Re: Deduction Question

Can I just say how crazy it makes me that LSAC uses this grammatical construction on its tests, and even crazier that it uses "if, but only if" to ALSO mean iff (aka double arrow)?

... Because while it may be standard in logic, both "if and only if" and "if but only if" are disparaged in legal usage by some leading authorities. See for instance Garner's Dictionary of Modern Legal Usage (Oxford University Press), p. 414.

But whatever... For the purposes of the LSAT, it means the ol' double arrow, or triple equals sign, or iff, or whatever.

Rrrrrrrrr.

brickman

Posts: 347
Joined: Mon Jun 29, 2009 2:59 am

### Re: Deduction Question

read the grouping games section of the LGB it is helpful for this particular issue. best of luck.

Curry

### Re: Deduction Question

If and only if means perfect causality.

If P, then Q.
P.
Therefore, Q.

Which is identical to

If Q, then P.
Q.
Therefore, P.