I have removed the statement "instance of chemical compound" from ethanol (Q153) [1]. 

A few proposals have been made in this thread about how -- or whether -- to use instance of (i.e. rdf:type, P31) to classify 'ethanol' and other chemical compounds, but there seems to be consensus that "instance of chemical compound" is not the way to do it.

Summary of proposals:

1. Do not use instance of for chemical compounds.  Such statements make Wikidata incompatible with many major scientific ontologies, like ChEBI, Gene Ontology and Disease Ontology, which use instance of as defined in the Relation Ontology (RO) [2].  Note that RO defines instances as particular things that have a unique location in space and time, whereas classes are universal, general entities which have particular instances.  Instances and classes are thus disjoint, so RO-based ontologies cannot have entities that have both instance of (rdf:type, P31) and subclass of (rdfs:subClassOf, P279) statements as is possible in OWL 2 DL via punning.
   
   
2. Use statements like "instance of type of chemical compound" for chemical compounds.  Doing so makes it easier to generate lists of chemical compounds, and is valid in OWL 2 DL -- it is metamodeling via punning.

Let's build consensus for how (or whether) we want to use instance of for chemical compounds before any mass edits to remove or replace the 14969 other "instance of chemical compound" claims [3] or adding statements like "instance of type of chemical compound" to ethanol.

Micru has a different proposal for how to model items, which incidentally does not represent "ethanol" as an instance [4].  However, that proposal is clearly a more radical vision for Wikidata, and probably warrants a separate thread for discussion.

Eric

https://www.wikidata.org/wiki/User:Emw

[1] Removal of "instance of chemical compound" from ethanol: https://www.wikidata.org/w/index.php?title=Q153&diff=162563849&oldid=162327014

[2] Barry Smith et al. (2005).  Relations in Biomedical Ontologies.  http://genomebiology.com/2005/6/5/r46
[3] All "instance of chemical compound" claims on Wikidata.  http://tools.wmflabs.org/wikidata-todo/autolist.html?q=claim[31:11173]
[4] "'ethanol' is no longer an instance, but a class".  https://lists.wikimedia.org/pipermail/wikidata-l/2014-October/004691.html

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