I'm looking for articles or books that discuss how we account for qualitative identity.
The distinction between qualitative and numerical identity is often drawn, but then the discussion tends to veer solely in favour of numerical.

What I want to know is what are the identity conditions of a book or a theory?
If a small child with no training clearly and cogently explains Pythagorean theorem, what is the nature of the identity statement we are making? On what do we base our judgement that x = y when x and y are concepts, stanzas or chords?

Presumably it is a qualitative identity, x and y share enough of the particular qualities that are required to make our judgement of identity true.
But what are the borderline cases? If the hypothetical monkey-on-a-typewriter were to bash out Romeo and Juliet with every 3rd word as 'Banana', would it still be Romeo and Juliet? If not why not? If so, how many words would have to be replaced to make it not so, and why?

Does anybody have any suggestions for reading material on these topics?

1 Answer 1


The literature on identity is enormous, so it would be easier to answer your question if you narrowed your wishes down a bit.

If you are a beginner to the field, a good starting point is the Stanford Encyclopedia of Philosophy article on Identity, which contains a reasonable bibliography, but is slanted towards the analytic tradition.

If you are interested in the Continental side of things, two classic texts are Heidegger's Identity and Difference and Deleuze's Difference and Repetition-- although you should be forewarned, both are difficult going.

Finally, if you are looking for some thought experiments to explore on your own, I'd suggest beginning with the Ship of Theseus.

  • These are great recommendations for numerical identity, but I've largely exhausted what I can find in relation to qualitative identity in the sources you've listed (in the analytic tradition at least). The sources you mention (Ship of Theseus in particular) deal with the diachronic identity of physical objects, rather than concepts, kinds or other abstract objects.
    – Sam Martin
    Jan 31, 2012 at 18:45
  • 1
    There isn't really a critical difference between physical objects and concepts in this regard, or between diachronic and spatial separation; the question of "is it the same ship, or a different ship?" remains. The identity condition of a text or a theorem will be different depending on the purpose of the exercise; from a literary perspective, two editions of the "same book" are identical if they contain the same text-- but to a book collector, two editions are not at all the same. These matters are, necessarily, contextual. Jan 31, 2012 at 20:20

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