理论

A theory divorced from practice is good for nothing.

In theory, your proposal may work, but in practice it will not.

Don't just talk about abstract principles.

theory in command

theoretical attainment

theoretical level

theoretical realm

theoretical value

theoretical chemistry/physics/biology

theoretical foundation/basis/support

theoretical contingent

theoretical innovation

separate theory from practice

link/unite theory with practice

integrate theory with practice; put theory into practice

confirm a theory

advance/present/propose/suggest a theory

test/verify a theory

construct/establish/found/frame/formulate a theory

He has his own reasons for doing so.

I have to argue this out with him.