Sentence:  You have heard him tell these young people that during his almost 50 years of service in the Congress he has seen the Kaisers and the Hitlers and the Mussolinis , the Tojos and Stalins and Khrushchevs , come and go and that we are passing on to them the freest Nation that mankind has ever <WSD>known</WSD> . 
Meaning of the word marked with <WSD> tags:  have firsthand knowledge of states, situations, emotions, or sensations