Willy does not play jazz

These arguments both infer a conclusion from two premises, but there is an important difference.

In argument A, each premise supports the conclusion individually. That is, PI is cited as a reason for C, and P2 is cited as another reason for C. One could argue from PI alone to C; one could also argue from P2 alone to C. By contrast, in argument B, neither premise supports C by itself. Neither PI nor P2 would, by itself, be a reason to accept C. Rather, they work together to support C.

Figure 6.2 Two premises supporting a conclusion individually
Figure 6.3 Two premises supporting a conclusion jointly

Argument A is represented as in Figure 6.2 and argument B is represented as in Figure 6.3.

Now these examples are very simple. The real value of argument trees emerges when they are used to represent more complex arguments with intermediate conclusions. For example:

P1) Consumption is increasing.

P2) The pound is weakening against other currencies.

C1) Inflation will increase.

P3) Whenever inflation increases, mortgage rates rise.

C2) Mortgage rates will rise.

P4) Whenever mortgage rates rise, the building trade suffers.

C3) The building trade will suffer.

Figure 6.4 Extended argument

The correct argument tree for this would be as shown in Figure 6.4. Note that this is not a completely reconstructed argument. PI and P2 do support CI, but we have not included the connecting premise or premises. The most plausible connecting premise would be of the form, 'If PI and P2, then CI'. We had the same sort of situation in the argument about Susan the marathon runner. The connecting premise was left implicit. As plausible as any connecting premise, in this case, would be something like

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