Existential ‘There’

Halliday & Matthiessen (2004: 119n):

… this there is a pronoun. The proportionality is:

the : that : it :: a(n) : one : there

Validity

Halliday & Matthiessen (2004: 117):

Semantics has nothing to do with truth; it is concerned with consensus about validity, and consensus is negotiated in dialogue.