If we split it into 2 sentences, The gentleman is John’s uncle. They say HE is one of the greatest scientists in the world. Because it is HE so we use WHO If it is HIM then we use WHOM. eg. The gentleman is John’s uncle. I know him. eg. The gentleman is John’s uncle, whom I know. Basically if it is the OBJECT of a verb (him/her) then we use whom. eg ’The girl who you described as lazy gave me this’ is WRONG ’The girl whom you described as lazy gave me this’ the original sentence = The girl gave me this. You described HER as lazy Cheers!!