Deductive reasoning is a logical process in which a conclusion is based on the concordance of multiple premises that are generally assumed to be true. Deductive reasoning is sometimes referred to as top-down logic.
Deductive reasoning relies on making logical premises and basing a conclusion around those premises. It does not rely on making inferences, then assuming those inferences to be true. Deductive reasoning is an important general skill for individuals of all types, because it will allow one to make an inference based on two or more pieces of information -- an inference which one can be relatively confident in.
Deductive reasoning is also a propositional logic in artificial intelligence (AI). Even though, given various rules and facts, an AI could use deductive reasoning, common sense AI is still a challenge.
Aristotle and deductive reasoning
The Greek philosopher Aristotle, who is considered the father of deductive reasoning, wrote the following classic example:
P1. All men are mortal.
P2. Socrates is a man.
- Therefore, Socrates is mortal.
Aristotle’s example is called a syllogism. A syllogism uses deductive reasoning to arrive at a conclusion that is based on two or more propositions that are assumed to be true. This is also called a premise premise conclusion argument. The premises of Aristotle's logical argument -- that all men are mortal and that Socrates is a man -- are self-evidently true. Because the premises establish that Socrates is an individual in a group whose members are all mortal, the inescapable conclusion is that Socrates must likewise be mortal. To correctly counter the conclusion of this argument, one must be able to disprove one of the premises.
Inductive vs. deductive reasoning
While deductive reasoning proceeds from general premises to a specific conclusion, inductive reasoning proceeds from specific premises to a general conclusion. While deductive reasoning is top-down logic, inductive reasoning is sometimes referred to as bottom-up logic.
Inductive reasoning relies on inferences made off of assumptions. For example, “the sun will rise tomorrow because the sun always rises in the morning.” Another example could be if a person has only ever seen white birds before, so they assume all birds are white. The conclusion of inductive reasoning is often based on the evidence given.