Inductive vs. Deductive Reasoning

Deductive Arguments are arguments that claim to provide complete support for the conclusion, i.e., arguments whose claim is that if the premises are true, the conclusion must be true. (Equvalently, we can say that deductive arguments are those whose claim is that it impossible to have true premises and false conclusion at the same time.)

Inductive Arguments are arguments that claim to provide some, but not complete, support for the conclusion. An important consequence of this definition is that with inductive arguments, no matter how strong they are, it is always possible for the conclusion to be false even though the premises are true.

Examples of Deductive Arguments
• All dogs are mammals. All mammals have kidneys. Therefore all dogs have kidneys.
• Since all squares are rectangles, and all rectangles have four sides, all squares have four sides.
• All chemists are smart, since chemists are scientists and all scientists are smart. (Note: Although the conclusion is probably false, the flaw in the argument is that one of the premises is presumably false; it remains true that if the premises are true, the conclusion must also be true.)
• Since all men are mortal, and Socrates is a man, Socrates is mortal.
• The sun is a star; the sun has planets; therefore some stars have planets.

Examples of Inductive Arguments
• All swans we have seen have been white; therefore all swans are white.
• All swans we have seen have been white; therefore the next swan we see will be white.
• All known planets travel about the sun in ellipitical orbits; therefore all planets travel about the sun in ellipitical orbits.
• Exploration of the surface of Mars has produced some surprising facts. Therefore exploration of the surface of Jupiter will produce some surprising facts.
• Since Chris is a good athlete, Chris's sister must be a good athlete also.

January 2, 1998