https://www.newadvent.org/fathers/04163.htm
The proverb 'a fight about the shadow of an ass.' is the ancient equivalent of the golden tooth of Fontenelle:
http://hoaxes.org/archive/permalink/the ... lden_tooth
One of the most famous texts by the French philosopher Fontenelle is entitled "The gold tooth". What is it about? The story takes place in 1593 in Silesia. A seven-year-old child, the age at which milk teeth are lost, suddenly has a gold tooth. The wise men, intrigued, immediately set to work to try to understand the miracle, multiplying the hypotheses together with books.
But a goldsmith who examines the child discovers that no miracle has happened: the object of the controversy was actually a gold leaf applied over a normal tooth. Fontenelle makes fun of the haste of the wise: "First they started making books and then they consulted the goldsmith".
Fontanelle comments on this anecdote regretting the fact that we look for the reasons for the facts we are talking about before verifying them, perhaps not realizing how paradoxical his operation is: to criticize the habit of commenting on non-existent facts, he himself invented a fable.
So, returning to Celsus, his point may be considered a precursor of the Fontenelle's point: the Jews and Christians dispute on the messianic status of Jesus, without before agreeing at least about the banal fact of the his existence.