Deduction

You can help students visualize valid and invalid deductive statements by identifying the key terms in each statement, reducing the statement to a syllogism, and assigning a letter to each term.  Then diagram the statement using a Euler diagram as shown below.

Valid syllogism:

All A is B (e.g. All mammals [A] breathe air [B])

All C is A (e.g. All whales [C] are mammals [A])

Therefore all C is A (e.g. all whales [C] breathe air [B]

Examples of possibilities for one invalid syllogism:

All A is B (e.g. all dictators [A] are people who rise to power aggressively when countries are in a state of depression [B])

All C is B (e.g. Sadam Hussein [C] is a person who rose to power aggressively when his country was in a state of depression [B])

Therefore all C is A (e.g. Sadam Hussein [C] is a dictator [A])