 Apart from the intrinsic interest of the subject, the paper marks a modification of Lakatos’s conception of factual novelty and hence a modification to the MSRP. For the earlier Lakatos, a fact counts as novel with respect to a research programme if it is not predicted by any of its rivals and if it is not already known. In WDCRPSP Lakatos accepts an amendment due to his co-author Elie Zahar. Zahar’s original problem was our old friend the Precession of Mercury. This was explained by Einstein’s programme—specifically the General Theory of Relativity—but not by Newton’s, and this was generally thought to count in Einstein’s favour. The difficulty is that in Lakatos’s lexicon the Precession of Mercury did not count as a novel fact. After all it had been known to astronomers for nearly a century. Thus, given the original version of the MSRP, the discovery that that the General Theory could explain the Precession of Mercury (whilst Newton’s theory could not) did not mean that Einstein’s programme was any more progressive than Newton’s. (That had to be argued on other grounds.) But this is such a counterintuitive result that it suggests a defect in the MSRP. Zahar’s modification is that a fact counts as a novel prediction with respect to a research programme if a) it is not predicted by any of the programme’s rivals and b) either it is not already known or if it is already known, the hard core of the programme was not devised to explain it.