Full Idea
The common end [of a ship], in which the parts conspire, is the same under all variations, and affords an easy transition of the imagination from one situation of the body to another.
Gist of Idea
The purpose of the ship makes it the same one through all variations
Source
David Hume (Treatise of Human Nature [1739], I.IV.6)
Book Reference
Hume,David: 'A Treatise of Human Nature', ed/tr. Selby-Bigge/Nidditch [OUP 1978], p.257
A Reaction
It is not true that a ship remains the same under ALL variations. Consider gradually changing a yacht into a racing powerboat. You might say the purpose is then changed, but the slight variations in a yacht can slightly change its purpose.