Comments inline

Paul Murray wrote:
...
How about:
An Occurrence is the reification of an individual's involvement in (entaglement with? presence at? relationship to?) an event. It reifies an "Event involvesIndividual Individual" fact.

The need for this construct is that we often need to say a number of additional things about an individual's involvement with (presence at) an event beyond simply assertin that there is some relationship. We need to say what tokens that individual left, what role that individual had (Predator? Prey? Parasite?), perhaps temporal or other limits of that particular individual at the event. Occurrence is the object to which these facts may be attached. An individual might meaningfully have more than one occurrence at an event - particularly in cases where events are part-of larger events, or where an individual somehow has multiple roles (hyenas chased away from their kill by a lion - or is it the other way around?).

To put it another way: "reification" = "tuple" = "association table" = "pulling a property out into an object". More or less.

Yes and I would add "=RDF graph node" to your list above.  It is a relationship made into something onto which we can slap an identifier and link other resources via object properties.  I would add that its nature is similar to that of dwc:Event (which you could say is a reification of an Occurrence's relationship to a Location and time) and a dwc:Identification (which you could say is a reification of the relationship between an "Individual" and the Taxon that somebody asserts that it represents).  None of these things (Occurrence, Event, or Identification) have a physical existence.  They exist because we need them to connect instances of other classes of resources.  We give them names that have something to do with the nature of the connection, but they really are just connections (or nodes if you wish).

Peter DeVries wrote:
> Wouldn't the individual be asserted to be an instance of a species "concept"
Defining an Individual this way assumes that one knows what taxon the Individual represents.  What about the Individual I've assigned the GUID http://bioimages.vanderbilt.edu/ind-baskauf/70858 ?  I don't really have any idea what it is.  That really shouldn't prevent me from assigning it a GUID and an rdf:type.  I suppose that you could do what Rich suggested which was to require at least one dwc:Identification at a level as high as "Life" but I would prefer to say that an Individual could have 0 to many dwc:Identifications, with "many" being the number of Taxa that people wish to assert that it represents.  That number is never fixed because somebody could always come along later and assert that it was something else, or even assert that it is the same taxon that somebody else previously had asserted. 

Richard Pyle wrote:
>
> I think the hardest part will be to define the allowable scope of “Individual”.  In my mind, it should at least span from single organism to multiple organisms up to
>colony and population; and can be circumscribed by any taxon concept (including “Life”).  I have been re-thinking whether “part” should be treated as a separate
> individual.  I used to think yes, but lately I’ve been thinking no.

I think that the answer to this question is related to the original definition of Paul's that started this thread (i.e. reification of relationships to create identifiable/linkable entities).  Back in October when you and I were butting heads on this topic, I think that the mistake was to try to demand that an Individual represent both a physical individual organism and the "reification" (my new favorite word) of an conceptual entity that connects an Identification to an Occurrence (a so-called TaxonomicallyHomogeneousUnit) [see Kevin Richard's comment at http://lists.tdwg.org/pipermail/tdwg-content/2010-November/001956.html which first made this distinction clearly].  In some cases, it is easy to define a resource as being both things (e.g. discrete organisms) but in others it's not (colonial organisms, clones, herds, tissue cultures, etc.).  After thinking about this for a long time I felt that the best thing to do was to cleanly separate the physical thing (a.k.a. a "token") from the conceptual thing (the reified entity onto which we can hang Identifications and Occurrence records).  If one wishes or if it is convenient, one could describe a particular resource as both the token and the conceptual thing (e.g. in the case of a discrete organism) but if it is more convenient one can identify the conceptual entity as a separate thing and identify any number of related tokens associated with this entity.  These related tokens could span the range from individual cells to tissues to discrete organisms to herds to populations and the relationships among them could be described using dcterms:hasPart or other appropriate object properties.  This was the approach we decided to take with darwin-sw (see http://code.google.com/p/darwin-sw/wiki/ClassIndividual http://code.google.com/p/darwin-sw/wiki/TokenIssues and http://code.google.com/p/darwin-sw/wiki/ClassToken for a more detailed description of the approach with diagrams and references).  This approach can handle these situations:
Richard Pyle wrote:
> Most people think of a coral head as an “individual”, but it’s really a colony of polyps.  Ants are usually curated as individual organisms, but can also be useful to collapse to colonies. 
Nico Cellinese wrote:
> I also agreed that an individual doesn't have to be whole, so in my opinion "parts" can also be considered 'the individual in question'. Similarly, I also agree that up to populations we can still talk about individuals.  However, it is important to be able to link "parts" to the whole individuals if needed, e.g. is_part_of.

Separate the abstract TaxonomicallyHomogeneousUnit from the physical tokens and this issue goes away:
Richard Pyle wrote:
> That is my old thinking.  My new thinking is that you abstract up to an
> whole organism when you have a part (e.g., tissue sample), when representing
> an Occurrence; just as you would abstract down to "Indidividual", when you
> what to represent Taxon-at-Event for an Occurrence.

In darwin-sw we did not attempt to define a real ontology of tokens that are physical parts or conglomerations of organisms (cells/tissues/discrete organisms/herds/populations/colonies/clones/etc.), but that would be a good thing to do.  It could be done in isolation from the definition of the "reified" TaxonomicallyHomogeneousUnit. 

Steve
-- 
Steven J. Baskauf, Ph.D., Senior Lecturer
Vanderbilt University Dept. of Biological Sciences

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