# Inference

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**Inferences** are steps in reasoning, moving from premises to logical consequences; etymologically, the word *infer* means to "carry forward". Inference is theoretically traditionally divided into deduction and induction, a distinction that in Europe dates at least to Aristotle (300s BCE). Deduction is inference deriving logical conclusions from premises known or assumed to be true, with the laws of valid inference being studied in logic. Induction is inference from particular evidence to a universal conclusion. A third type of inference is sometimes distinguished, notably by Charles Sanders Peirce, contradistinguishing abduction from induction.

Various fields study how inference is done in practice. Human inference (i.e. how humans draw conclusions) is traditionally studied within the fields of logic, argumentation studies, and

## Definition

The process by which a conclusion is inferred from multiple

This definition is disputable (due to its lack of clarity. Ref: Oxford English dictionary: "induction ... 3. Logic the inference of a general law from particular instances." ^{[}*clarification needed*]) The definition given thus applies only when the "conclusion" is general.

Two possible definitions of "inference" are:

- A conclusion reached on the basis of evidence and reasoning.
- The process of reaching such a conclusion.

## Examples

### Example for definition #1

Ancient Greek philosophers defined a number of syllogisms, correct three part inferences, that can be used as building blocks for more complex reasoning. We begin with a famous example:

- All humans are mortal.
- All Greeks are humans.
- All Greeks are mortal.

The reader can check that the premises and conclusion are true, but logic is concerned with inference: does the truth of the conclusion follow from that of the premises?

The validity of an inference depends on the form of the inference. That is, the word "valid" does not refer to the truth of the premises or the conclusion, but rather to the form of the inference. An inference can be valid even if the parts are false, and can be invalid even if some parts are true. But a valid form with true premises will always have a true conclusion.

For example, consider the form of the following

- All meat comes from animals.
- All beef is meat.
- Therefore, all beef comes from animals.

If the premises are true, then the conclusion is necessarily true, too.

Now we turn to an invalid form.

- All A are B.
- All C are B.
- Therefore, all C are A.

To show that this form is invalid, we demonstrate how it can lead from true premises to a false conclusion.

- All apples are fruit. (True)
- All bananas are fruit. (True)
- Therefore, all bananas are apples. (False)

A valid argument with a false premise may lead to a false conclusion, (this and the following examples do not follow the Greek syllogism):

- All tall people are French. (False)
- John Lennon was tall. (True)
- Therefore, John Lennon was French. (False)

When a valid argument is used to derive a false conclusion from a false premise, the inference is valid because it follows the form of a correct inference.

A valid argument can also be used to derive a true conclusion from a false premise:

- All tall people are musicians. (Valid, False)
- John Lennon was tall. (Valid, True)
- Therefore, John Lennon was a musician. (Valid, True)

In this case we have one false premise and one true premise where a true conclusion has been inferred.

### Example for definition #2

Evidence: It is the early 1950s and you are an American stationed in the

Knowns: The Soviet Union is a

Explanation: In a

## Incorrect inference

An incorrect inference is known as a fallacy. Philosophers who study informal logic have compiled large lists of them, and cognitive psychologists have documented many biases in human reasoning that favor incorrect reasoning.

## Applications

### Inference engines

AI systems first provided automated logical inference and these were once extremely popular research topics, leading to industrial applications under the form of

An inference system's job is to extend a knowledge base automatically. The knowledge base (KB) is a set of propositions that represent what the system knows about the world. Several techniques can be used by that system to extend KB by means of valid inferences. An additional requirement is that the conclusions the system arrives at are relevant to its task.

Additionally, the term 'inference' has also been applied to the process of generating predictions from trained

#### Prolog engine

Let us return to our Socrates syllogism. We enter into our Knowledge Base the following piece of code:

mortal(X) :- man(X). man(socrates).

( Here * :-* can be read as "if". Generally, if

*P Q*(if P then Q) then in Prolog we would code

*Q*(Q if P).)

**:-**PThis states that all men are mortal and that Socrates is a man. Now we can ask the Prolog system about Socrates:

?- mortal(socrates).

(where *?-* signifies a query: Can *mortal(socrates).* be deduced from the KB using the rules)
gives the answer "Yes".

On the other hand, asking the Prolog system the following:

?- mortal(plato).

gives the answer "No".

This is because

Prolog can be used for vastly more complicated inference tasks. See the corresponding article for further examples.

### Semantic web

Recently automatic reasoners found in

### Bayesian statistics and probability logic

Philosophers and scientists who follow the

Bayesians identify probabilities with degrees of beliefs, with certainly true propositions having probability 1, and certainly false propositions having probability 0. To say that "it's going to rain tomorrow" has a 0.9 probability is to say that you consider the possibility of rain tomorrow as extremely likely.

Through the rules of probability, the probability of a conclusion and of alternatives can be calculated. The best explanation is most often identified with the most probable (see

### Fuzzy logic

### Non-monotonic logic

^{[1]}

A relation of inference is

By contrast, everyday reasoning is mostly non-monotonic because it involves risk: we jump to conclusions from deductively insufficient premises. We know when it is worth or even necessary (e.g. in medical diagnosis) to take the risk. Yet we are also aware that such inference is defeasible—that new information may undermine old conclusions. Various kinds of defeasible but remarkably successful inference have traditionally captured the attention of philosophers (theories of induction, Peirce's theory of abduction, inference to the best explanation, etc.). More recently logicians have begun to approach the phenomenon from a formal point of view. The result is a large body of theories at the interface of philosophy, logic and artificial intelligence.

## See also

*A priori*and*a posteriori*– Two types of knowledge, justification, or argument- Abductive reasoning – Inference seeking the simplest and most likely explanation
- Deductive reasoning – Form of reasoning
- Inductive reasoning – Method of logical reasoning
- Entailment– Relationship where one statement follows from another
- Epilogism
- Analogy – Cognitive process of transferring information or meaning from a particular subject to another
- Axiom system– Mathematical term; concerning axioms used to derive theorems
- Axiom – Statement that is taken to be true

- Immediate inference – Logical inference from a single statement
- Inferential programming
- Inquiry – Any process that has the aim of augmenting knowledge, resolving doubt, or solving a problem
- Logic – Study of correct reasoning
- Logic of information
- Logical assertion– Statement in a metalanguage
- Logical graph– Type of diagrammatic notation for propositional logic
- Rule of inference – Systematic logical process capable of deriving a conclusion from hypotheses
- List of rules of inference
- Theorem – In mathematics, a statement that has been proven
- Transduction (machine learning) – Type of statistical inference

## References

**^**Fuhrmann, André.*Nonmonotonic Logic*(PDF). Archived from the original (PDF) on 9 December 2003.

## Further reading

- Hacking, Ian (2001).
*An Introduction to Probability and Inductive Logic*. Cambridge University Press.ISBN 978-0-521-77501-4. - Jaynes, Edwin Thompson (2003).
*Probability Theory: The Logic of Science*. Cambridge University Press.ISBN 978-0-521-59271-0. Archived from the originalon 2004-10-11. Retrieved 2004-11-29. - .
- .

**Inductive inference:**

- Carnap, Rudolf; Jeffrey, Richard C., eds. (1971).
*Studies in Inductive Logic and Probability*. Vol. 1. The University of California Press. - Jeffrey, Richard C., ed. (1980).
*Studies in Inductive Logic and Probability*. Vol. 2. The University of California Press.ISBN 9780520038264. - Angluin, Dana (1976).
*An Application of the Theory of Computational Complexity to the Study of Inductive Inference*(Ph.D.). University of California at Berkeley. - Angluin, Dana (1980). "Inductive Inference of Formal Languages from Positive Data".
*Information and Control*.**45**(2): 117–135. . - Angluin, Dana; Smith, Carl H. (Sep 1983). "Inductive Inference: Theory and Methods" (PDF).
*Computing Surveys*.**15**(3): 237–269.S2CID 3209224. - Gabbay, Dov M.; Hartmann, Stephan; Woods, John, eds. (2009).
*Inductive Logic*. Handbook of the History of Logic. Vol. 10. Elsevier. . - Goodman, Nelson (1983).
*Fact, Fiction, and Forecast*. Harvard University Press.ISBN 9780674290716.

**Abductive inference:**

- O'Rourke, P.; Josephson, J., eds. (1997).
*Automated abduction: Inference to the best explanation*. AAAI Press. - Psillos, Stathis (2009). "An Explorer upon Untrodden Ground". In Gabbay, Dov M.; Hartmann, Stephan; Woods, John (eds.).
*An Explorer upon Untrodden Ground: Peirce on Abduction*(PDF). Handbook of the History of Logic. Vol. 10. Elsevier. pp. 117–152.ISBN 978-0-444-52936-7. - Ray, Oliver (Dec 2005).
*Hybrid Abductive Inductive Learning*(Ph.D.). University of London, Imperial College. .

**Psychological investigations about human reasoning:**

**deductive:**- Johnson-Laird, Philip Nicholas; Byrne, Ruth M. J. (1992).
*Deduction*. Erlbaum. - Byrne, Ruth M. J.; S2CID 657803. Archived from the original(PDF) on 2014-04-07. Retrieved 2013-08-09.
- Knauff, Markus; Fangmeier, Thomas; Ruff, Christian C.; (PDF) on 2015-05-18. Retrieved 2013-08-09.
- Johnson-Laird, Philip N. (1995). Gazzaniga, M. S. (ed.).
*Mental Models, Deductive Reasoning, and the Brain*(PDF). MIT Press. pp. 999–1008. - Khemlani, Sangeet; Johnson-Laird, P. N. (2008). "Illusory Inferences about Embedded Disjunctions" (PDF).
*Proceedings of the 30th Annual Conference of the Cognitive Science Society. Washington/DC*. pp. 2128–2133.

- Johnson-Laird, Philip Nicholas; Byrne, Ruth M. J. (1992).
**statistical:**- McCloy, Rachel; Byrne, Ruth M. J.; S2CID 7741180. Archived from the original(PDF) on 2015-05-18. Retrieved 2013-08-09.
- .,

- McCloy, Rachel; Byrne, Ruth M. J.;
**analogical:****spatial:**- Jahn, Georg; Knauff, Markus; .
- Knauff, Markus; .
- Waltz, James A.; Knowlton, Barbara J.; Holyoak, Keith J.; Boone, Kyle B.; Mishkin, Fred S.; de Menezes Santos, Marcia; Thomas, Carmen R.; Miller, Bruce L. (Mar 1999). "A System for Relational Reasoning in Human Prefrontal Cortex".
*Psychological Science*.**10**(2): 119–125.S2CID 44019775.

**moral:**

## External links

- Inference at PhilPapers
- Inference example and definition
- Inference at the Indiana Philosophy Ontology Project