Assessing Logical Strength
Assessing Arguments with Independent Premises

1. When there are independent premises within a single step -- that is, when two or more arrows converge on the same conclusion -- the argument is at least as strong as the strongest component.

Example:

In one of his essays, Carl Sagan makes the following argument (numbers have been inserted):

"(1) Science is based on experiment, (2) on a willingness to challenge old dogma, (3) on an openness to see the universe as it really is. Accordingly, (4) science sometimes requires courage." (Broca's Brain)

The conclusion here is obviously 4, and 1 through 3 appear to be independent premises for it. What is the connection between courage and doing experiments? This argument seems extremely weak. Challenging old dogmas could require courage, so the connection between 2 and 4 seems stronger. Finally, what is the connection between "an openness to see the universe as it really is" and courage? Once again, the link with courage is weak.

Here we have two very weak links and a fairly strong one. This argument is at least as strong as its strongest link.


Independent Premises | Dependent Premises | Multistep Arguments

Assessing Logical Strength

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