"this theory states" is correct and usable in written English.
It is typically used to introduce a theory or idea being discussed in a text. For example, "This theory states that natural selection is a primary force in evolutionary adaptation."
This theory states that markets efficiently incorporate all public information, which consequently renders beating the market impossible.
This theory states that the impact of an asteroid on Earth may have triggered the extinction event by ejecting a huge quantity of rock debris into the atmosphere, enshrouding Earth in darkness for several months or longer.
This theory states that a bolide (meteorite or comet) impact may have triggered the extinction event by ejecting a huge quantity of rock debris into the atmosphere, enshrouding Earth in darkness for several months or longer.
Since the 18 current and former city tax assessors charged in the case were accused of taking bribes in exchange for lowering the assessments, and the property taxes, for landlords, this theory states that the city lost out on property taxes that it was due.
Our original interpretation of this theory states that high-quality stocks adhere to these repetitive patterns most consistently.
This theory states, roughly, that each substance has an infinitely complex complete concept that specifies everything that will ever happen to that substance.
Steihnübel rejects this theory, stating that the king preserved the whole kingdom for himself.
This theory stated that the electron may exhibit the properties of both a wave and a particle.
Over many generations this has led to the menopause, the theory states.
Distilled to its essence, this groundbreaking theory states that Oswalt is more effective when he is performing well.
On this theory, conscious states are just those mental states of which the subject has higher-order, quasi-perceptual awareness (see the entry on higher-order theories of consciousness).
