Deductive reasoning means starting with a general rule or principle and using it to figure out specific facts. When you use deductive reasoning, you work from what you know must be true to discover what else has to be true.

Here's how it works: If you know that all mammals have backbones, and you learn that whales are mammals, you can deduce (figure out through deduction) that whales must have backbones. You didn't need to examine a whale to know this. The logic led you there.

Deductive reasoning is like following a chain of locked doors where each key opens the next one. Detectives use it when solving mysteries: if they know the burglar entered through the window, and only someone tall could reach that window, they can deduce the burglar must be tall. Mathematicians use it constantly. If you know that 5 + 7 = 12, and someone asks what 7 + 5 equals, you can deduce it's also 12 because addition works the same forwards and backwards.

The opposite is inductive reasoning, where you observe specific examples and make a general conclusion. If you notice that every dog you meet has a tail, you might inductively reason that all dogs have tails (though you can't be completely certain without checking every dog). Deductive reasoning, when done correctly, gives you certainty. If your starting facts are true and your logic is sound, your conclusion must be true.