Definition of "class"

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In 2.5  Universals and classes, the term "class" is used but not clearly 
defined:  

"Universals have instances, which are in every case particulars (entities 
located in space and time). Universals also have extensions, which we can think 
of (set-theoretically) as collections of their instances. 
Such extensions fall outside the scope of this specification, but it is 
important for the understanding of BFO that the distinction is recognized. It 
implies further distinctions not only between universals and their extensions 
but also between universals and classes in general, including arbitrary classes 
such as: {the moon, Napoleon, redness}.  "

It would be helpful to clarify whether "extension" mean "class". I would also 
propose to introduce a (functional) relation like "extension of", which links 
classes with universals.

Original issue reported on code.google.com by steschu@gmail.com on 14 May 2013 at 8:53

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I have understood classes to mean also logical combinations of universals that 
are not themselves universals. For examples (Apples OR Oranges). I think 
extension here is not the same as class. Universals (as well as classes) have 
extensions which may change from moment to moment. The phrase "classes such as: 
{the moon, Napoleon, redness}" means, the class with extension {the moon, 
Napoleon, redness}, presumably whenever the members exist, since, as far as I 
know, it does not make sense to have, in BFO, either a class nor universal 
whose extensions contain, at a moment, things that don't exists at the time 
that the class or universal exists.

I imagine one could extend the notion of class to allow this, but then the 
notion of extension would be confusing, meaning something different for classes 
than for universals.

Part of the confusion in the above is in the phrase "collections of their 
instances", since instantiation is time dependent we don't know if it means "at 
a time", or whether it means "ever". In the latter case it would be a strange 
entity as it would include all instance in history but also in the future.

Original comment by alanruttenberg@gmail.com on 15 May 2013 at 10:28

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Also, what is the use case for having the relation "extension of".

Original comment by alanruttenberg@gmail.com on 15 May 2013 at 10:28

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Original comment by alanruttenberg@gmail.com on 15 May 2013 at 10:35

• Added labels: Milestone-BFO2-Release

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Original comment by alanruttenberg@gmail.com on 5 Jun 2013 at 6:25

• Changed state: Accepted
• Removed labels: Milestone-BFO2-Release