Three realities or degrees of the theory, moving respectively from one minimum requirement for reporting (observing) adequacy to the ultimate goal of explanatory adequacy through the mediation of descriptive adequacy.
This three-threshold difference can best be explained by the following example. Let’s take the theory of language. This theory has observational adequacy if it provides us with a growing string of words, but only if the competent speaker of the language intuitively recognizes it as grammatically correct. It would be correct to say that a theory possessing descriptive adequacy (or being descriptively adequate) would also provide resources to describe why within a language a particular string of words is capable of producing meaning and another producing nonsense.
Theory must also provide these resources in an intuitively convincing way to the competent user of the language.
But the ultimate goal of any linguistic study seeking scientific status is to go beyond pure observation and description to explanatory adequacy.
Related: Erklärung
The theory of language has explanatory adequacy when it is able to identify the mechanisms or means by which meaningful or grammatical sentences are created.
