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
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,
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