Multi-Step Arguments and Multiple Arguments to the Same Conclusion

§2.4

In §1.3 above we saw that there are multi-step arguments, where a premise of one argument is supported by a further argument. You need to be on the lookout for this sort of structure when mapping. Here’s an example:

Jane’s visit must have been over a weekend, since she spent two full days here, and she wouldn’t have been able to do so during the week. But Rob wasn’t in town, so the visit had to be on the first weekend in July, since that’s the only one when he wasn’t here.

Map 10:

The first sentence of the passage gives us Argument A, with Proposition 1 as its conclusion, and the second sentence then gives us the remaining propositions in Argument B.

It is not uncommon in such multi-step arguments for some of the propositions to be left implicit. For example, here’s another way in which someone might express the same argument mapped above.

Jane wouldn’t have been able to spend two whole days here during the week. But the only weekend when Rob was out of town was the first one in July, so her visit must have been over that weekend.

Here Propositions 1, 2 and 4 (from Map 10) are left implicit, but it is reasonably clear that the author of the passage intended the argument expressed by that map.