Reasonableness
The concept of reasonableness has two related meanings in law and political theory:

 As a legal norm, it is used "for the assessment of such matters as actions, decisions, and persons, rules and institutions, [and] also arguments and judgments." As a regulative idea, it "requires... that all factors that might be relevant in answering a practical question be considered and... that they be assembled in a correct relation to each other in order to justify [a judgement]." Reasonableness should not be conflated with rationality. Political theory 
Reasonableness has been discussed by political thinkers such as John Rawls (in his 1993 Political Liberalism), T. M. Scanlon, Brian Barry and Georg Henrik von Wright. Law 
The notion of "reasonableness" is omnipresent in European law, and has also affected "international treaties and general customs". Examples of its use can be found in canon and medieval law, suggesting roots going back to Ancient Rome. Standards and doctrines 
Standards and doctrines requiring reasonableness include:

Reasonability
Reasonable accommodation
Reasonable act
Reasonable appearance of danger
Reasonable care
Reasonable cause or reasonable and probable cause
Reasonable and competent support
Reasonable creature
Reasonable danger
Reasonable diligence
Reasonable doubt
Reasonable expectation (Legitimate expectation is sometimes called reasonable expectation.) Reasonable facilities
Reasonable fitness
Reasonable mind
Reasonable and non-discriminatory licensing
Reasonable person or reasonable man
Reasonable portion
Reasonable possibility
Reasonably practicable
Reasonable and probable damage
Reasonable and probable grounds
Reasonable provocation
Reasonable prudence
Reasonable question
Reasonable rates
Reasonable regulation
Reasonable right of way
Reasonable skill
Reasonable suspicion
Reasonable time
Reasonable use
Reasonable wear and tear excepted
Subjective and objective standard of reasonableness

Constitutional and administrative law 
In constitutional and administrative law, reasonableness is a lens through which courts examine the constitutionality or lawfulness of legislation and regulation. According to Paul Craig, it is "concerned with review of the weight and balance accorded by the primary decision-maker to factors that have been or can be deemed relevant in pursuit of a prima facie allowable purpose". Common law 
Examples of reasonableness standards in common law jurisdictions include:

 Reasonableness simpliciter and patent unreasonableness (repealed in 2008) in Canadian law
 Wednesbury unreasonableness in English law
 Wednesbury unreasonableness in Singaporean law

Mixed jurisdictions 

 Reasonableness cause in Israeli law (עילת הסבירות)

Reasonability

Reasonability is a legal term. The scale of reasonability represents a quintessential element of modern judicial systems and is particularly important in the context of international disputes and conflicts of laws issues. The concept is founded on the notion that all parties should be held to a reasonable standard of conduct and has become embedded in a number of international conventions such as the UNIDROIT principles and the CISG. The concept of reasonability is applicable to Roman law. See also 
 Reason

References

Further reading

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Administrative law
Constitutional law
Legal doctrines and principles
Political philosophy