principle

principle — noun

1. a general scientific or logical rule that explains how a system, process, or are

1.nounC1
Definition

a general scientific or logical rule that explains how a system, process, or area of knowledge operates — for example, the principle of supply and demand in economics, or Archimedes' principle in physics

Example

The principle of supply and demand is central to modern economics.

principle of [field]: a core rule in a discipline

Archimedes' principle explains why some objects float in water.

Common collocations
  • basic principle
  • guiding principle
  • first principle
  • work on the same principle
  • principle of [field]
Synonyms
  • law

    a principle that is universally observed and often formulated mathematically (e.g., the law of gravity)

  • rule

    a more general term for a guideline; less fundamental than a scientific principle

  • tenet

    a principle held to be true by a group or organisation, especially in religion or politics

Usage Note

Often followed by "of" plus a field of knowledge, as in "the principle of supply and demand" or "principles of design."

Common Mistakes

The principal of supply and demand drives prices.
The principle of supply and demand drives prices.
'principal' means the head of a school or most important; 'principle' means a rule or idea.

2. used to say that something is generally accepted as a good idea in theory, even

2.nounC2
Definition

used to say that something is generally accepted as a good idea in theory, even if the practical details are not yet settled or fully carried out

Example

The committee agreed to the plan in principle but requested more details.

collocation: agree in principle

In principle, the proposal makes sense — the real difficulty is putting it into action.

contrast: in principle vs. putting into action

Common collocations
  • agree in principle
  • accept in principle
  • possible in principle
Synonyms
  • theoretically

    more informal and broader in use; 'in principle' implies the general idea is accepted, whereas 'theoretically' leaves its acceptance open

  • in theory

    nearly interchangeable with 'in principle,' though 'in principle' often carries a stronger hint of agreement

Antonyms
  • in practice

    the real-world application, as opposed to the general idea

Grammar Patterns

in principle

Usage Note

Used only as a fixed phrase — "in principle." Do not use "by principle" or "on principle" to mean the same thing. "On principle" belongs to sense 3 (see idiom "as a matter of principle").

Common Mistakes

By principle, I support the idea.
In principle, I support the idea.
the fixed phrase is 'in principle,' not 'by principle.'

3. a firm personal belief about what is morally right or wrong that guides the way

3.nounC2
Definition

a firm personal belief about what is morally right or wrong that guides the way you live and behave

Example

Obi refused to lie to the customer because it went against his principles.

idiom: go against one's principles

Devika left the company as a matter of principle after they fired a colleague unfairly.

idiom: as a matter of principle

Common collocations
  • against one's principles
  • as a matter of principle
  • on principle
  • strong principles
  • a person of principle
  • have principles
Synonyms
  • morals

    closely related; 'principles' are more consciously held and often publicly stated, whereas 'morals' can be internal and instinctive

  • values

    broader than principles; values include what you consider important in life, while principles are specific rules of right and wrong

  • ethics

    a formal system of moral principles, often in professional contexts (e.g., medical ethics)

Usage Note

Often uncountable when referring to someone's overall moral character (e.g., "a man of principle") and countable when referring to specific beliefs (e.g., "she has strong principles about honesty").

Common Mistakes

I don't eat meat in principle of animals.
I don't eat meat as a matter of principle.' or 'I don't eat meat on principle.
'in principle of' is not a correct phrase.
Idioms

against one's principles

idiom

morally unacceptable to someone, because it conflicts with their personal standards of right and wrong

Taking a bribe was completely against Andrei's principles as a public servant.

It would be against my principles to accept money for a review I did not honestly write.

as a matter of principle

idiom

because of your personal moral beliefs, not for practical, financial, or emotional reasons

Maeve returned the extra change to the cashier as a matter of principle.

Hamza refused the job offer as a matter of principle when he learned the company had broken safety laws.

in principle

idiom

Used to say that something is possible or acceptable as a general idea, even if it has not been done or agreed in detail.

The committee accepted the plan in principle but asked for changes.

In principle, the new policy should reduce costs for everyone.

Word family

  • principal
    adjectiveB2
  • principled
    adjectiveC1
  • unprincipled
    adjectiveC2

Synonym discussion

Sense: noun/1

Principle, rule, law, tenet, and doctrine all describe a statement of how something works or how people should behave, but they differ in scope and authority. A principle is a fundamental truth that serves as the foundation for a system (e.g., the principle of democracy). A rule is a more specific guideline that can be imposed or agreed upon, and it may have exceptions. A law is a principle so well established that it is treated as universally true or legally binding. A tenet is a principle held by a particular group, especially in religion or politics — it implies shared belief rather than external proof. A doctrine is a set of official principles taught by an institution, such as a church or government. For example: "Einstein's theory is built on the principle that the speed of light is constant" (principle = foundation). Compare: "Church doctrine forbids this practice" (doctrine = authoritative teaching from an institution).

rule ↗law ↗tenetdoctrine ↗

Common collocations

Nouns

  • principle + of [field]
  • principle + of [morality]
  • principle + of [physics/economics/design]

Adverbs

Phrases

Prepositions

Etymology

LatinFrenchEnglish

The word "principle" entered English through Old French "principe," from Latin "principium" meaning "a beginning, origin, or foundation." In Latin, "princeps" means "first" or "leader" (from "primus" = first + "capere" = to take), so a principle is literally what comes first — the starting point from which other ideas follow.