Full Idea
The classical definition of validity counts an argument as valid if and only if the conclusion does in fact follow from the premises, whether or not the argument contains any demonstration of this fact.
Gist of Idea
Validity is a conclusion following for premises, even if there is no proof
Source
David Bostock (Intermediate Logic [1997], 1.2)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.5
A Reaction
Hence validity is given by |= rather than by |-. A common example is 'it is red so it is coloured', which seems true but beyond proof. In the absence of formal proof, you wonder whether validity is merely a psychological notion.