"What can be said at all can be said clearly, and what we cannot talk about we must pass over in silence"
- Ludwig Wittgenstein
These words beautifully explain the concept behind the so called "Private Language Argument", one of the most important topics of 20th century philosophy. The seeds of this thought were immanent in the ideas of John Locke(1632-1704) and Gottlob Frege(1848-1925), but was fully developed by Ludwig Wittgenstein(1889-1952), perhaps the greatest 20th century philosopher.
In the simplest words, the "argument" is that "the concept of a private language is improbable". By "private language" what is meant is "a language that is private to an individual and hence understandable by nobody else".
This simple looking argument has deep philosophical consequences. On the surface level, the immediate implication is that all language is socially constructed and can never be a subjective or private phenomenon. But the more important consequence is deeper, and has been explained in two different ways.
The first and more popular of the two, popularized initially by Rudolf Carnap(1891-1970) is that, the "argument" invalidates or at least looks with great suspicion, the validity of the "fully subjective nature of consciousness" (often called in philosophical terms as qualia). And this in turn has major implications for the "Mind-Body Problem" in philosophy, and hence religion and spirituality. As a result non-objective and non-empirical sources of language and thought as well as most of metaphysics is considered invalid.
With regard to "Mind-Body Problem", Carnap's interpretation of "Private Language Argument" shifts the advantage to the empirical/materialistic side. In other words the "argument" supports the primality of the "body" with respect to "mind", and more or less rejects metaphysics and mysticism as valid sources of knowledge.
The second interpretation, which seems to be the one favored by Wittgenstein himself, is that, "Private Language Argument", doesn't invalidate metaphysics and subjectivity, rather it enforces a limit on the "natural sciences" and particularly rationalism. Wittgenstein believed that philosophy should start where "natural sciences" end.