In my last post I promised to explain the reasoning behind the rules of validity that relate to the distribution of terms. Recall that a term is distributed in a statement when it refers to the entire extension of the term. This implies, as we saw, that the subjects of universal statements and the predicates of negative statements are distributed.
One related rule of validity says this:
A valid syllogism must distribute in its premise any term distributed in the conclusion.
This syllogism, for example, breaks this rule:
All mammals are warm-blooded creatures.
No birds are mammals.
∴ No birds are warm-blooded creatures.
This syllogism distributes the major term “warm-blooded creatures” in the conclusion. The claim being made is that the entire extension of warm-blooded creatures does not include any birds. But in the major premise, “All mammals are warm-blooded creatures,” that term is not distributed. All we know is that some warm-blooded creatures are mammals, but no claim is being made about all of them in the premise. This means that the conclusion claims more than the premises. But in a valid syllogism, the conclusion cannot claim more than the premises. Thus, this syllogism is invalid.
The fallacy being made here is called illicit major, because the major term in the conclusion goes beyond the major term in the premise. An example of illicit minor would be this:
All globes are spheres.
Some balls are spheres.
∴ All balls are globes.
You should be able to convince yourself that the conclusion makes a claim about all balls, but the minor premise does not.