The phrase theory of mind can be used in several ways. There are the specific theories of mind attributable to individuals.
Each of us knows by introspection that we understand that others have beliefs, desires and intentions that are different from our own and we infer that all humans with normal minds share this cognitive ability.
There has also been speculation that certain humans fail to progress through the normal cognitive developmental stages that lead to acquisition of a theory of mind.
With the advent of brain imaging techniques, particular brain regions that seem to be important for theory of mind have been identified.
The theory of mind that normal children develop appears to be that other people have different knowledge from them, but process their knowledge in the same way that they would.