How do you define a collection?

The DLH thread has made me think about the notion of "what is a collection" and has raised some philosophical questions.
Is a collection the set of all coins you have owned or only the ones you've owned at a given point and sold at once?
If you were to produce a coffee table book of your collection, is it inappropriate to include all of the coins you've historically owned alongside the ones you own at the end?
It can be beneficial to recycle funds for those of us who don't have unlimited budgets but there's a particular point where it becomes absurd (i.e. if you own only one coin, sell it, use the funds to buy one other, sell it, etc.) That's less of a collection and more just hunting and pecking but is it okay if you do this in groups of 20 (i.e. a box of 20)? 10? 5?
If a collection is going to be judged by the X key coins, would it "count" to just buy those key coins first, then eventually sell them and buy the rest of the breadth of the collection?
Just food for thought, but I'm curious as to what other members think.
Comments
A collection is just what you've collected, not what you've sold to get what you've put in your collection. A coffee table book of your collection should only have what you're keeping, unless it's a history of how you got to what you've got now. Once sold, it's no longer part of your collection.
The great Art Dealer Lord Joseph Duveen (d.1939) often told his top clients "An accumulation is NEVER a collection!". He felt there needs to be a clear theme or direction or linkage between the individual pieces.
By that definition, very few of us have "a collection". I know mine is an accumulation.
My accumulation of similar objects I would call my collection period.
don't sweat by thinking too hard 
To various degrees it is still a collection of stuffs......I won't put too much thoughts to it as far as how the collection's come about
Be Happy
...and I rather not define happiness either
A bunch of metal disks that you own and like.
My problem is I don't want to do the hard work of paring the accumulation into something more viable. When I have tried to do so in the past, I don't miss 95% of it but then I let go of something that I miss and/or regret letting go of.
The emotion of Happiness is too tricky.
As a math person it must pain you to exclude the empty set/collection there.
The hobby of getting up close to history and owning peace’s of history.
There's obviously a whole multi-
dimensional spectrum of the term "collection."
How do you define an Athlete or Singer or Artist?
Calling them one is a start, but only a start.
I agree that a Collection has some sort of cohesiveness, some unifying theme that the person doing it can articulate, and "interested parties" can appreciate.
My Dansco 7070 album, or my 90% complete set of Browning variety large sized quarters might qualify for most neutral observers.
Someone else's $1000 face bag of 90% junk silver might, or might not.
Small eagle, we all agree, without exception, that you have a Collection. In fact, you have a COLLECTION!!
But, the coins we've sold clearly are Not "in it" any more, right?
Liberty: Parent of Science & Industry