Almost every website that talks about it says, “Inductive reasoning is a bottom up” approach while “deductive is a top down approach”.
Can anyone explain to me the THE DIFFERENCE? What makes these two forms of reasoning so different? Examples are always appreciated as well.
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To most philopshers:
Deduction is when you make 100% confident chains of logic. Like a proof of there being infinite prime numbers.
These deductions are allowed to have conditionals or hypthethetials or unfounded assumptions in them, but *if* those things are true, then the deduced conclusion is also surely true.
However Induction is when you use logic or reasoning to make statements that seem pretty likely due to evidence, but aren’t totally 100% solid. For instance, suppose that you go to hospital, and you happen to know that 95% of the nurses in this hospital are women. It would be *inductive* reasoning to say “The nurse I get will be a woman.” – that probably is true, but not certain.
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For a scientist, it is similar, but we might use these in a particular context.
For something very simple:
“If gravity is real, then this pen will fall when I drop it. I assume gravity is real, so therefore the pen will fall.” is deductive reasoning.
However, let’s say I do some experiments. “I dropped 100 pens, and they always fell. I therefore conclude that dropped pens will always fall.” that is inductive reasoning.
Maybe the scientist could be wrong. The next pen might not fall. Pens might not always fall. But have 100 pens fall each time you tested it is evidence that this is just how the world works.
Furthermore, “Since pens will always fall, and gravity predicts that pens would fall, gravity must be real.” is also inductive reasoning.
The scientist could be wrong again. Maybe something other than gravity is causing the pens to fall when dropped. The pens dropping is good evidence for gravity, but doesn’t totally 100% prove it.
Deductive:
All men are mortal – Socrates is a man – thus Socrates is a mortal.
Inductive: The sun came up everyday this week – therefore, it’ll be coming up tomorrow.
Inductive is using what you know is true to know more things that are true. Deductive is using what you know is false to determine what must be true.
ELI5:when you know for sure how things work and what is true in an area: like you know things about triangles or addition, or even numbers under 40 and over 0. Then you can use deduction because you know the rules of the domain. Triangles have three corners and three sides no more no less… the new shape has 4 corners and 3 sides (some how) so it is not a triangle (since it breaks a rule you know for certain.)
On the other hand, when you know some examples, even a lot of examples, but you do not know for certain the forces at work then you cannot deduce. The universe is bigger than your knowledge set. So then you use inductive reasoning: which works, until it does not: “The farmer feeds me every morning… This is a morning… Hey Farmer, I wonder how I’m supposed to eat an axe?!”
Inductive reasoning is the sun will come up tomorrow because it has come up every day before. We can not generalize to all suns and planets since we have not observed them.
Deductive reasoning is the sun will come up tomorrow based on observation of celestial bodies and the earth will spin to face the sun. We can generalize to all suns and planets.
What they’re describing is the relationship between **Specific Facts (‘Top’)**, and **Generalizations (‘Bottom’).**
**Deductive** reasoning is going from top to bottom. You take established facts, and then make a general conclusion from them. It’s probably what you think of when you think about “basic logic problems”.
“Well, *all* dogs naturally have ears, and *all* Golden Retrievers are a type of dog. That must mean that all golden retrievers naturally have ears.” **Specific > General**
**Inductive** reasoning is going the opposite direction. It’s taking in general points, and getting specific facts from them. You probably understand this better as “pattern recognition”.
“Well, a **pent**agon has 5 sides, **hex**agon has 6 sides, and an **oct**agon has 8 sides. That must mean a **non**agon has 9 sides, and a **dec**agon has 10 sides.” **General > Specific**. (Penta = “five”, “hexa” = “six”, etc. if you don’t know greek)
Each type is useful for something different.
Deductive reasoning is powerful assuming your presumed facts are correct,but hard facts aren’t too common, and general answers aren’t always helpful. Inductive reasoning on the other hand is generally a lot easier to implement (it gives specific answers), but it’s a bit more prone to error because generalizations can have unexpected exceptions.
Fun Fact: Sherlock Holmes is famously called “The Master of Deduction”, despite the fact that he only ever uses Inductive Reasoning in most of his incarnations.
I have taught the difference between inductive and deductive reasoning as well, but with some of these explanations, I wonder if someone with an undergrad would even understand them…
To understand the top down / bottom up analogy, think of the problem as a tower, with the answer/conclusion at the bottom; and the facts/clues stacked on top.
Now consider one piece of the tower is missing, how do you figure out its shape?
Deductive reasoning/top down approach is you look at all the clues and reach a conclusion of the right shape. Sherlock is a great example of this kind of reasoning. It does have its weaknesses, as it assumes all of the facts are correct and interpreted correctly.
Inductive reasoning/bottom up is you look at the conclusion and the clues you do have and figure out the missing clue. It still suffers from the same weaknesses of veracity of information.
In an example, let’s say you have a crime scene: three suspects, 1 gun, 1 bullet hole in a wall. You’re trying to figure out who made that shot.
If all you have is the bullet hole and the gun, you can use inductive reasoning to determine that the shot was made from a lower angle, so the shortest suspect is the most likely to have made the shot.
However, if you have more information, such as gun shot residue on the tallest suspect you can use deductive reasoning to say that even though it’s a lower angled shot, maybe the tallest suspect was crouching. However, this conclusion might not be valid if there is missing or contradictory information.
Neither method is right or wrong, it depends on what you’re looking for and what you’re starting with. Often you’ll use a combination of both to confirm that both the facts and conclusions support each other.
Deductive reasoning: The sprinklers are running in the lawn right now. Sprinklers spread water which makes things wet. Therefore the will be wet.
Or simpler A Causes B. A is happening to C. Therefore C will be B. There’s no fault in that logic, it can’t possibly be not true.
Inductive reasoning: The lawn is wet. Sprinklers make things wet. Therefore the sprinkler must’ve run earlier to make the lawn wet.
In other words C is B. A Causes B. Therefore A must’ve happened to C.
This logic is flawed because there are many things that could make the lawn wet. It could’ve rained, someone could’ve emptied a cooler out onto it, etc. With inductive reasoning you’re trying to build backwards and it can often be flawed.
At the end of every Sherlock Holmes book, Sherlock tells a funny joke that no one understands.
He tells Watson that he used deductive logic. That’s the joke, but no one gets it. Instead I had teachers telling me that Sherlock was an example of deductive logic. Yikes.
Sherlock doesn’t use deductive logic because any idiot can do that. It is basically figuring out what must have come next. For example, if a dog is hungry then it will eat food.
What Sherlock does instead is look at the final picture and figure out how it happened without knowing what went first. He looks at a million options of how it could have gone down and his mind is able to figure out the pathway it took to get there. It’s partly logic but partly intuition. You feel the answer and then you check to see if it makes sense. Like when kids since math problems with the Guess and Check method.
Hope this helps
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