Deductive reasoning is one of the two basic forms of valid reasoning. While inductive reasoning argues from the particular to the general, deductive reasoning argues from the general to a specific instance. The basic idea is that if something is true of a class of things in general, this truth applies to all legitimate members of that class. The key, then, is to be able to properly identify members of the class. Miscategorizing will result in invalid conclusions.

Examples of deductive reasoning may be both subtle and time-saving. For example, Be careful of that wasp: it might sting. is based on the logic that wasps as a class have stingers; therefore each individual wasp will have a stinger. This conclusion is freeing in that we do not have to examine each and every wasp we ever encounter to ascertain what characteristics it may have. Because of the validity of deductive reasoning, we may make an assumption that is both useful and efficient.

One of the most common and useful forms of deductive reasoning is the syllogism. The syllogism is a specific form of argument that has three easy steps.

• 1. Every X has the characteristic Y.
• 2. This thing is X.
• 3. Therefore, this thing has the characteristic Y.

Let's look at what each step in the deductive reasoning process means.

1. The first step names a definitive property of X, whatever X is.

Examples :

• Every triangle has three sides.
• Frogs are amphibians.
• A standard major league baseball game has 9 innings.

2. The second step proclaims that a particular item/person fits into the category that has been composed.

Examples:

• The figure I drew is a triangle.
• The coqui is a frog native to Puerto Rico.
• That was a standard major league ballgame.

3. The third step applies deductive reasoning, connecting the general truth stated in step 1 to the particular case mentioned in the second step.

Examples:

• This triangle I drew has three sides.
• The coqui is an amphibian.
• That ballgame had 9 innings.

Creating a syllogism is a good way to test your deductive reasoning to ensure that it is valid.