Full Idea
We call a belief true when it is belief in a true proposition, ..but it is to propositions that the primary formal meanings of 'truth' and 'falsehood' apply.
Gist of Idea
In its primary and formal sense, 'true' applies to propositions, not beliefs
Source
Bertrand Russell (On Propositions: What they are, and Meaning [1919], §IV)
Book Reference
Russell,Bertrand: 'Logic and Knowledge', ed/tr. Marsh,Robert Charles [Routledge 1956], p.320
A Reaction
I think this is wrong. A proposition such as 'it is raining' would need a date-and-time stamp to be a candidate for truth, and an indexical statement such as 'I am ill' would need to be asserted by a person. Of course, books can contain unread truths.