Full Idea
A relation is internal if it is unthinkable that its object should not possess it. (This shade of blue and that one stand, eo ipso, in the internal relation of lighter to darker. It is unthinkable that these two objects should not stand in this relation).
Clarification
'Eo ipso' (Latin) means by that very fact
Gist of Idea
A relation is internal if it is unthinkable that its object should not possess it
Source
Ludwig Wittgenstein (Tractatus Logico-Philosophicus [1921], 4.123)
Book Reference
Wittgenstein,Ludwig: 'Tractatus Logico-Philosophicus (Pears)', ed/tr. Pears,D. /McGuinness,B. [RKP 1961], p.27
A Reaction
An epistemological definition. If only one shade of blue existed, would it still have this internal relation? Are things therefore full of potential internal relations with non-existent things?