The anecdote runs like this: in 1801, in his habilitation thesis De orbitis planetarum, Hegel proved out of pure concepts that there could be only seven planets – and in that very year the eighth was discovered. Ever since, it has counted as proof of what comes of a philosopher prescribing to nature.
What the thesis says
At the close Hegel reports a series of numbers from Plato’s Timaeus and sets it against the then current Titius–Bode series, from which an undiscovered planet between Mars and Jupiter had been inferred. His sentence is conditional: if the series of the Timaeus were correct, then no room would remain between the fourth and the fifth planet. Whether it is correct he leaves open. A proof that there could be no further planets does not stand there.
And the chronology
Giuseppe Piazzi discovered Ceres on 1 January 1801; Hegel’s thesis appeared in August of the same year. Whether it was a planet at all remained open for years – in November 1802 William Herschel took Ceres for a comet. Today it counts as a dwarf planet. The point of the anecdote, that a philosopher was overtaken by reality, presupposes a finding that did not then exist.
What remains true in it
In 1954 Bertrand Beaumont showed in Mind that the legend has „no foundation whatsoever in Hegel’s texts“; Wolfgang Neuser worked out the conditional character of the passage. This relieves Hegel of this anecdote – not of the question how his philosophy of nature stands as a whole. That is a question of its own, and it is treated here.
The anecdote is instructive in another respect: it has circulated for over a hundred years without anyone opening the thesis. One can study in it how a reputation comes about.
Sources. G. W. F. Hegel, Dissertatio philosophica de orbitis planetarum, Jena 1801 · Bertrand Beaumont, „Hegel and the Seven Planets“, in: Mind 63 (1954) · Wolfgang Neuser, introduction to the edition of the dissertation, 1986 · Jon Stewart (ed.), The Hegel Myths and Legends, 1996.