The phrase "a viable theory" is correct and usable in written English.
It can be used when discussing a theory that is practical, feasible, or capable of being successfully implemented or tested.
Example: "After extensive research, we concluded that the proposed model is indeed a viable theory for explaining the observed phenomena."
Alternatives: "a feasible theory" or "a workable theory".
As such, the Grokster case remains a viable theory for holding defendants accountable.
A viable theory of river behaviour must reconcile the various processes that occur at different scales.
While issues remain, these results are significant progress towards a viable theory for the 2 3 kHz emissions.
Notwithstanding the reservations expressed in Tuttle, the Court of Appeals "continued to believe" that gross negligence in police training was "a viable theory of municipal liability" under § 1983.
The lack of a viable theory for studying the sources, distribution, and effect of odors has necessitated the accurate measurement of odors from environmental compartments.
It's certainly a viable theory, given that services such as Spotify and Last.fm have proved hugely successful, but have yet to be mimicked by iTunes, which doesn't allow streaming of full songs.
Blocking offers some of the strongest evidence that learning depends not on mere temporal contiguity, but on contingency, and is now viewed as an essential phenomenon for any viable theory of learning to explain.
"I think that's a very viable theory," a senior government official said.
(And this is probably all viable theories).
The standard model of cosmology does a lot of things right, and it is very difficult to create a viable alternative theory of gravity that can compete with it.
The Discovery Institute has from its beginning claimed it would in short order get actual scientists to consider intelligent design as a viable scientific theory, by publishing peer-reviewed articles in the leading science journals.
