Full Idea
The 'best candidate' theories get into difficulty because it is assumed that there is a single uniquely correct answer to the question 'Which is the real ship?'
Gist of Idea
The 'best candidate' theories mistakenly assume there is one answer to 'Which is the real ship?'
Source
Peter Simons (Parts [1987], 5.5)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.204
A Reaction
My own example supports Simons. If Theseus discards the old planks as rubbish, then his smart new ship is the original. But if he steals his own ship (to evade insurance regulations) by substituting a plank at a time, the removed planks are the original.