Full Idea
We can know the properties of the relations required to preserve the correspondence between sense-data and reality, but we cannot know the nature of the terms between which the relations hold.
Gist of Idea
Because we depend on correspondence, we know relations better than we know the items that relate
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 3)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.16
A Reaction
Thus Russell always puts great emphasis on relations in his metaphysics. I would say that he is right, and that what he calls the 'nature of the terms' are essences, and that these are knowable, by inference and explanation.