Tarski's material adequacy condition, also known as Convention T, holds that any viable theory of truth must entail, for every sentence "P", a sentence of the following form (known as "form (T)"): (1) "P" is true if, and only if, P. For example, (2) 'snow is white' is true if and only if snow is white.
The reason they look trivial is that the object language and the metalanguage are both English; here is an example where the object language is German and the metalanguage is English: (3) 'Schnee ist weiß' is true if and only if snow is white.
It is important to note that as Tarski originally formulated it, this theory applies only to formal languages, cf.
Tarski developed the theory to give an inductive definition of truth as follows.
Tarski's semantic conception of truth plays an important role in modern logic and also in contemporary philosophy of language.