7-2,10

7-3,4

7-4,9

7-9,4

7-15,4

7-2, 10. Let's see . . . If we've got juice at the distributor, the coil isn't defective, and if the coil isn't defective, then the problem is in the ignition switch. So the problem is in the ignition switch.

Premises: If we've got juice at the distributor, the coil isn't defective. If the coil isn't defective, then the problem is in the ignition switch. Unstated premise: We've got juice at the distributor. Conclusion: The problem is in the ignition switch. The unstated premise is easier to spot once you realize that the premises are dependent on one another.

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7-3, 4. When blue jays are breeding, they become very aggressive. Consequently, scrub jays, which are very similar to blue jays, may also be expected to be aggressive when they're breeding.

Conclusion: Scrub jays may be expected to be aggressive when they're breeding. Be careful with the clause "which are very similar to blue jays." This functions as a premise in the argument, not part of the conclusion; without it among the premises, the argument isn't valid.

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7-4, 9. If you drive too fast, you're more likely to get a ticket. You're also more likely to get into an accident. So you shouldn't drive too fast.

Independent. Some repetition of words in the two premises ("you're more likely to get") may obscure the logical independence of these premises. What matters is not whether getting into an accident leaves you more likely to get a ticket but whether the two have to occur together to give you a reason not to drive too fast.

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7-9, 4. He'll drive recklessly only if he's upset, and he's not upset.

Unstated conclusion: He won't drive recklessly. It might be tempting to treat "he's not upset" as the conclusion here and look for an unstated premise to make that argument valid. But it isn't possible. No claim will produce a valid argument with "he's not upset" for its conclusion.

Suppose you try the following. He'll drive recklessly only if he's upset. He won't drive recklessly. Therefore (conclusion) he's not upset. "Isn't that just as good?" you might ask. "I'm supplying exactly the same claim as a premise that you can supply as a conclusion, so it has to be just as plausible." As a claim it's just as plausible; but it is not just as good at making the argument valid. For this alternative argument is invalid, as Chapter 9 will make clear.

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7-15, 4. They really ought to build a new airport. It would attract more business to the area, not to mention the fact that the old airport is overcrowded and dangerous.

One judgment call here: whether to break the last premise stated into two: The old airport is overcrowded, and the old airport is dangerous. Given that an airport may become dangerous for reasons aside from its overcrowding, these are really separate points. The speaker is giving three independent reasons for accepting the conclusion.