Full Idea
Hegel is reputed to have claimed to have deduced on a priori grounds that the number of planets is exactly five.
Gist of Idea
Hegel reputedly claimed to know a priori that there are five planets
Source
report of Georg W.F.Hegel (works [1812]) by Hartry Field - Recent Debates on the A Priori 1
Book Reference
'Oxford Studies in Epistemology Vol. 1', ed/tr. Gendler,R/Hawthorne,J [OUP 2004], p.69
A Reaction
Even if this is a wicked travesty of Hegel, it will do nicely to represent the extremes of claims to a priori synthetic knowledge. Field doesn't offer any evidence. I would love it to be true.