Full Idea
The word 'know' has two senses: the first is 'knowledge of truths', which is opposed to error, applies to judgements, and is knowing that something; the second is 'acquaintance', and is knowledge of things, particularly of sense-data.
Gist of Idea
Knowledge of truths applies to judgements; knowledge by acquaintance applies to sensations and things
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 4)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.23
A Reaction
We can also add procedural knowledge ('knowing how'). The question for Russell is whether his 'knowledge by acquaintance' can ever qualify as knowledge on its own, without the intrusion of judgements. Does perception necessarily have content?