One of the most influential scientists and mathematicians ever, Newton was a key figure in the scientific revolution. Barrow taught him at Cambridge and recommended him as his own successor. Halley was one of his keenest supporters – his zealous enthusiasm encouraged Newton to publish his Principia Mathematica. Newton met Huygens in London, told his former friend Flamsteed to stick to observation (not theory), and tried to discredit Leibniz (corresponding via Oldenburg as intermediary). He corresponded with Boyle about alchemy, Pepys about gambling odds, and Hooke (whose criticisms hurt) about optics. Cheselden advised against operating when Newton was dying.