Re: Entity and Identity

Brian wrote:

>On Jul 27, 7:24 pm, rp_at_raampje.(none) (Reinier Post) wrote:
>> Brian wrote:
>>
>> [...]
>> >It may be
>> >splitting hairs, but there is a distinct difference between 'identity'
>> >and 'the identity' in that 'identity' is a binary relation between
>> >objects in the universe that denotes /is identical to/, but 'the
>> >identity' of an object is that essential property (unary relation)
>> >which distinguishes it from all other objects (its haecceity) and
>> >which is embodied by an object identifier or by a proper name (in the
>> >logical sense).
>>
>> I think this is utterly mistaken, regardless of whether you're referring
>> to logic or to or OO programming.  Identity is never a relationship
>> between objects, but between identifiers that denote (refer to) objects.
>
>I suggest you bone up on Leibniz.

That's a bit difficult, my Latin is extremely poor. But I think I've identified the source of this misunderstanding: page 146 of 'Relational database: selected writings' (Addison-Wesley 1989) by C. Date.

>> And haecceity is not a property.
>
>Yes it is. Is not the sum of all qualitative properties also a
>property?

It depends on what you mean by that. The potential problem is not in considering a sum of properties as a property, but in the 'all'.

If you define the haeccity of an entity type as the collection of all qualitative properties we distinguish for that object type, then of course haeccity can be regarded as a property of such objects. It just becomes another way of saying that all properties of a relation form a key. I doubt this is what you mean.

If you define it as all qualitative properties that can conceivably be distinguish for a particular object type, then you start running into circles: for every property you consider that adds distinctions between objects, you will need to decide whether the distinction is an essential characteristic of the object type itself, or of an object type that is related to the object type you're describing. This is a modeling decision, not something that can be extracted from Mother Nature.

-- 
Reinier