identity and identifiers



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Joaquin Miller (joaquin@acm.org)
Sun, 27 Feb 2000 17:59:34 -0600


Perdita: thanks for the careful answer about total order and the thoughtful comments on my other puzzle. At 10:56 AM 2/27/00, Perdita Stevens wrote: >JM> What i am thinking is that there may be sets with >JM> elements that are distinct, but can not be distinguished by their >JM> properties. Consider electrons. Or identical (yes: identical, but not >JM> the same) balls in a (excuse me) bag.] > >Objects, unlike values of basic types, have identity as well as state, >which is why we get this idea of there being objects in the same state but >with different identities. But the point about things being equal is that >you can't distinguish them in any way. yes. but, by the way: i make a hard line distinction between 'identity' and 'identifier.' [side issue: likewise between identity and equality, as do you. (notice: this is a different, but closely related, meaning of 'identity.' and notice: the 'identical' in "identical balls" above has a quite different meaning: having the same value of all properties or something like that. "My identical twin.") but back to the subject.] The balls in the bag clearly have identity (the quality of a thing that it is itself and not some other thing), but they do not have identifiers. Although the balls are distinct and each has identity, they are (the ones in my example thought experiment) indistinguishable: you can't distinguish them in any way, one from another. By that i mean, if i give you one, and then take it back and give you one again, you can not distinguish between the following two cases: a) the second is the same ball as the first b) it is a different ball. I admit this is not an interesting case for OCL, which is why i made the first message private. ........ Of course, if i gave you two at the same time, you could distinguish them, one from the other. But you can expand on the experiment yourself ... (i give you two, you place them in (some) order on a rack, you give them back, i give you two again: are these the same two? if so, place them on the rack in the same order. if not. place them on the rack using the same total ordering you used the first time.) (i place two balls in front of you. you close your eyes for ten seconds...) no fair bending the corners of the cards. ==================================================== "at night, all cats are black." at night cats can not be distinguished, one from another, by the property, color. objects can never be distinguished by the quality, identity. all objects have identity. that is, each object is itself, and not some other object. having this quality affords no way to distinguish objects, one from another. ....... objects can (in the right circumstances) be distinguished one from another if they happen to have identifiers, because an identifier, RM-ODP teaches us, is "An unambiguous name, in a given naming context." [2-12.2] ======================================================= [In my world] the quality, identity, is easily distinguished from the quality, having an identifier. Cordially, Joaquin Miller Chief Architect Financial Systems Architects mailto:joaquin@acm.org +1 (510) 336-2545


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