in theory
Talking about something in principle
Use in theory for an idea that should work logically, even if reality may differ.
in theory, everyone has equal access.
In, of, with, between or on theory?
Different prepositions with theory express different meanings. Some are fixed expressions, while others depend on the wider sentence.
Quick answer
Common combinations with theory
The fixed phrase in theory means “in principle.” Theory of introduces a subject or explanation, as in theory of evolution. Between is used to compare theories. On theory is possible when meaning “about theory” in academic discussion. With theory is usually an adjunct, not a fixed combination.
in theory, the plan should work.
She studies the theory of language learning.
The debate between theories continued for years.
She studies the theory of language learning.
The debate between theories continued for years.
Choose the pattern by meaning
These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.
theory of + subject
Naming a field or explanation
Use theory of to identify what the theory concerns.
the theory of relativity
a theory of motivation
between + theories
Comparing theories
Use between when discussing differences or relationships among theories.
the conflict between theories of learning
on theory
Meaning about theoretical ideas
On theory appears in academic contexts to mean “about theory” or “concerning theory.”
a lecture on theory and practice
with + theory
Adding theory as a related element
With theory is not a fixed phrase with theory. It usually means accompanied by theory or involving theory.
a course with theory and lab work
See the difference in context
The same subject can take different constructions because each sentence answers a different question.
Construction
Example
Meaning
in theory
In theory, the process is simple.
in principle or according to an idea
theory of
She wrote a book on the theory of art.
a theory connected to a subject
with theory
The workshop combines practice with theory.
theory is an accompanying element
between theories
Researchers distinguished between theories of memory.
showing comparison or separation
on theory
We had a seminar on theory.
about theoretical concepts
Common mistakes
Some combinations appear in real text but do not mean what learners often assume.
Using on theory instead of in theory
For “in principle,” use the fixed phrase in theory.
✕ On theory, it should succeed. ✓ In theory, it should succeed.Using of theory after a general noun
Theory of usually needs a specific subject after it.
✕ She studies the theory. ✓ She studies the theory of communication.Confusing with theory and theory of
With theory means accompanied by theory, not a type of theory.
✕ a course with economics ✓ a theory of economicsUsing between with only one theory
Between normally connects two or more items.
✕ between the theory ✓ between two theoriesQuick practice
Choose the expression that matches the intended meaning.
1. ___, the new system should reduce delays.
2. They discussed the theory ___ evolution.
3. The professor gave a lecture ___ theory and method.
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Frequently asked questions
Is in theory a fixed expression?
Yes. It commonly means “in principle” or “according to an idea.”
Can I say on theory?
Yes, especially in academic English when meaning “about theory.”
Is with theory a standard collocation?
Not usually. With theory often functions as a separate phrase meaning accompanied by theory.
Related preposition guides
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