"primary theories" is correct and usable in written English.
It is typically used to refer to the main or most important theories in a particular subject or field. Example: "The primary theories of economics suggest that supply and demand drive market prices."
But two primary theories have been identified.
Here are the two primary theories.
Drawing on Kant, Schopenhauer and Freud, Mr. Malphurs identified three primary theories of laughter (superiority, incongruity and relief).
Here, we instantiate the primary theories of numerical representation in computational models and compare simulated performance to human data.
Pluralism and neo-corporatism are the two primary theories that have been put forward to explain interest group influence on public policy.
The three primary theories protecting consumers and imposing greater duties on sellers are contract theory, due-care theory, and strict-liability theory.
The Party Decides, for example, has been the primary theory driving how political analysts have thought about presidential campaigns.
The primary theory is that girls might be more honest in reporting injuries — though experts are confident that many girls, just like boys, hide injuries either because they are not aware of the risks or because they simply do not want to miss playing time.
Primary theory for single-phase flow in fracture was developed (Warren and Root 1963).
The primary theory about the astonishing rally being put forward by investors on social media is that bitcoin will soon benefit from big institutional money injections via the introduction of the first BTC futures products.
The primary theory is related to increased excitability of the cerebral cortex and abnormal control of pain neurons in the trigeminal nucleus of the brainstem.
