Halley’s part in the conception, development, printing and publication of the
Principia
, as seen from Newton’s side, is well known and well documented,1 and to that I have nothing to add. Without Halley, the stimulus, the critic, the supporter, editor and publisher, there would have been no
Principia
, or at least no published
Principia
as we now have it. Newton would probably have remained in relative obscurity in Cambridge and be known to us for his mathematics and optics but perhaps not as an outstanding figure in the history of science. Without Halley we should not have had Newton’s grand conception of how physical science should be pursued, the conception that still guides us. Although well established, the public history of still raises questions. It runs as follows. In the evening of 24 January 1684, at the Royal Society, Wren, Hooke and Halley discussed Halley’s demonstration that Kepler’s third law implied that the attraction of the Sun upon the planets was as the inverse of the square of the distance from them, and Wren offered to give books to the value of 40 shillings to whomever of Hooke and Halley could first show (before the end of March) that the inverse square law led to an elliptical orbit. March came and went. Wren’s books were not claimed, but not until August did Halley call on Newton in Cambridge. Why so long a delay? All three seem to have appreciated that the question was very important, but more than four months passed from the end of March before Halley went to Cambridge. Hooke would hardly have admitted his failure by asking Newton, and Wren was no doubt much occupied with building St Paul’s and otherwise, but Halley might surely have gone at his first opportunity. In fact it seems that he did.