Syntax is a branch of linguistics.

He is majoring in syntax at university.

Category theory is a branch of mathematics that studies structures and their relationships.

He is an expert in probability theory.

Einstein's theory of relativity is one of the important theories in physics.

A DD debate is unfolding between him and her.

He strongly advocated the hokushin theory.

He strongly advocated the theory of southward advancement.

He believes in teleology, thinking that there is a meaning in every event.

He is writing a thesis on the yellow peril.