However, it’s incorrect: sharks are not fish–they are mammals.

All birds have claws, and so do all lizards. But that doesn’t

mean that lizards are birds.

All apes have noses, and so do all humans. But that

doesn’t mean that all humans are apes. (Only humans

who are big and stupid are apes!)

Deductive reasoning is the kind of reasoning where the conclusion is necessitated by previously known premises. If the premises are true then the conclusion must be true. For instance, beginning with the premises “sharks are fish” and “all fish have fins”, you may conclude that “sharks have fins”. This is distinguished from inductive reasoning and abductive reasoning where inferences can be made with some likelihood but never with complete certainty.

Deductive reasoning is dependent on its premises. That is, a false premise can possibly lead to a false result, and inconclusive premises will also yield an inconclusive conclusion.

Contrast it with the famous example of the logical fallacy, “post hoc, ergo propter hoc,” or “after this, therefore because of this.” A moment’s reflection on any syllogism that proceeds from the premise that B followed A in time to the conclusion that therefore B must have been caused by A makes its absurdity self-evident.