Introduction to Logic: Inference

An inference is a process of deriving conclusions based on the evidence. It essentially involves using observation and background knowledge as well as other known premises to determine a conclusion that makes sense.

Examples of Inference

1. Ade arrives at home at 5:00. He knows his mother does not leave work until 6. He also sees that the lights are off in their house. Ade can infer that his mother is not yet home.
2. The floor is covered in shreds of newspaper and Hassan’s dog has newsprint on his paws. It can be inferred that Hassan’s dog ate the newspaper.
3. Chinedu works at a pet store and owns four cats, a lizard, a dog and a rabbit. It can be inferred that Chinedu is a pet lover.
4. Berries are poisonous to all dogs . You can Infer that berries are poisonous for your dog
5. My throat is sore and my nose is running , You can infer that I Probably have a Cold.

Types of Inference

There are two essential types of inference.

Deduction

Deduction (or “deductive inference”) is an inference based on logical certainty. It usually starts from a general principle and then infers something about specific cases.

“Grapes are poisonous to all dogs” – “I Shouldn’t give my Dog Grapes”.

This allows you to infer that grapes are poisonous for your dog, too. If the premise is true then the conclusion has to be true. There’s no other possibility. Notice, however, that this doesn’t really tell you anything new: once you say “grapes are poisonous to all dogs,” you already know that grapes are poisonous for your specific dog. Deduction has the advantage of certainty, but it doesn’t generate new knowledge.

Induction

Induction (or “inductive inference”) is an inference based on probability. It usually starts from specific information and then infers the more general principle.

“For the last two years, Amanda has woken up at 8am every day” – “Amanda will wake up at 8am”

This allows you to infer that Amanda will probably wake up at 8am tomorrow, too. You would probably be right, and it’s a reasonable inference but it’s not certain! Tomorrow could be the first day that Amanda decides to sleep in. Despite this uncertainty, however, induction does offer the possibility of predicting future events and creating new knowledge.

Inference and Observation

An inference starts from a premise (like a piece of evidence) and then moves beyond it. But what if you see the evidence for yourself? Do you need to make any inferences then?

Example

“I saw Mary walk into the supermarket the other day.”

This is a direct observation. It doesn’t seem to involve any inferences. But if you look with a careful, skeptical eye, you’ll see that it contains many inferences — what did you really see?

“I saw someone who looked like Mary walk into the supermarket the other day.”

It’s entirely possible that you made a mistake! It’s easy to mistake people on the street for people you know, so you can’t be entirely sure that you saw what you think you saw. The person could even have been a robot. Or you could have hallucinated the whole thing!

Of course, this is not the sort of thing you really need to worry about — 99% of the time, you’re correct about what you’re seeing. The point is just that observations are never 100% reliable, and always involve a certain amount of inference.

References

1. Inference: Examples and Definition
2. Examples of Inference