Deductive Reasoning
After general laws have been established, either by human experience or accepted inductive reasoning, they may be cited as applying to any particular case under consideration. This passing from the general law to the particular instance is deductive reasoning. Deductive reasoning has a regular form called the syllogism.
Major premise. All men are mortal.
Minor premise. Socrates is a man.
Conclusion. Therefore, Socrates is mortal.
If the three parts of a syllogism are correct it has absolute convincing power. Most attempts to disprove its statement attack the first two statements. Although it carries such an air of certainty it is likely to many errors in use. An error like this is common:
All horses are animals.
All cows are animals.
Therefore, all cows are horses.
Explain the fallacy in this syllogism.
Quite as frequently the incorrect syllogism is of this kind.
The edge of a stream is a bank.
A bank is a financial institution.
Therefore, the edge of a stream is a financial institution.
You will comment upon this that its evident silliness would prevent any speaker from using such a form in serious argument. But recall that in the discussion of any idea a term may get its meaning slightly changed. In that slight change of meaning lurks the error illustrated here, ready to lead to false reasoning and weakening of the argument. Certain words of common use are likely to such shifting meanings—republic, equality, representative, monarchy, socialistic. Any doubtful passage in which such an error is suspected should be reduced to its syllogistic form to be tested for accuracy.
A representative of the people must vote always as they would vote.
A Congressman is a representative of the people.
Therefore, Congressmen must vote always as the people who elect them would vote.
Is not the expression, representative of the people, here used in two different senses?
When an argument is delivered, one of the premises—being a statement which the speaker assumes everyone will admit as true—is sometimes omitted. This shortened form is called an enthymeme.
Smith will be a successful civil engineer for he is a superior mathematician.
Supply the missing premise. Which is it?
In the bald, simple forms here set down, the syllogism and enthymeme are hardly suited to delivery in speeches. They must be amplified, explained, emphasized, in order to serve a real purpose. The following represent better the way a speaker uses deductive reasoning.
The appointing power is vested in the President and Senate; this is the general rule of the Constitution. The removing power is part of the appointing power; it cannot be separated from the rest.
Daniel Webster: The Appointing and Removing Power, 1835
Then Daniel Webster stated in rather extended form the conclusion that the Senate should share in the removing proceedings.
Sir, those who espouse the doctrines of nullification reject, as it seems to me, the first great principle of all republican liberty; that is, that the majority must govern. In matters of common concern, the judgment of a majority must stand as the judgment of the whole.
Daniel Webster: Reply to Calhoun, 1853
Then, he argues, as these revenue laws were passed by a majority, they must be obeyed in South Carolina.