Assessing Logical Strength
Assessing Multistep Arguments

When an argument has more than one step, it can be no stronger than its weakest step. A chain can be no stronger than its weakest link.

Example: Consider the following argument:

1. Identical twins are more alike in intelligence than fraternal twins.
2. Identical twins are genetically identical, whereas fraternal twins are not.
3. Intelligence is genetically determined to some extent.
4. Therefore, intelligence cannot be increased by training or education.

In this argument, 1 and 2 jointly support 3, which supports 4, the conclusion.

In the first step, the premises offer fairly strong evidence for the conclusion. In the absence of further evidence, 3 is the more reasonable conclusion to draw from 1 and 2.

However, the second step, from 3 to 4, is much weaker. The fact that a trait has a genetic basis doesn't mean that it is unaffected by the environment.


Independent Premises | Dependent Premises | Multistep Arguments

Assessing Logical Strength

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