From: "A Dreamer of Pictures: Neil Young" by David Downing. (page 54) -

"There were good musical reasons for adding SOMEONE. First and foremost they needed some help to take their music out on the road. ... For an electric band they would need either another guitarist or a keyboard player, preferably someone who was both. The first choice was Stevie Winwood, at that time between legs of Blind Faith's one, immensely lucrative tour of the US. Stills and drummer Dallas Taylor went to see him in England, trudging through knee-deep mud to Winwood's country cottage, only to find that this particular member of rock's first supergroup had already had enough of such aggregations."

From the CSN retrospective boxed set:

Stephen Stills: "Stevie Winwood was always my favorite singer, that blue-eyed soul sound. It had been our ambition from the start to convince him to join Crosby, Stills and Nash - I wanted an organ player who could sing the blues. He was exceptionally kind to me, but everytime I trudged across the moors to see him, he was always occupied. I didn't think there was anyone more shy than me – the phrase painfully shy was invented for this man. We recorded "Dear Mr. Fantasy" a number of times, beginning in 1970. It was intended as a homage. Hats off, Stevie."