I thought about offering the dictionary definition, but that would be cheating, so:

If you can point at it, touch it, hear it, smell it, taste it, speak of it, think about it, be scared of it, or worship it, it is one.

There may be more of them that don't fit into any of these categories. But if there are, we cannot yet concieve of them. But when we can, they will be also :)

I can point at the sky. I can hear music. I can see colours. I can think about life and can be scared of death. I can worship some principle of nature.

Makeshifts last the longest.

And you don't classify all of those..er..things as "things"?

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Examine what is said, not who speaks.

"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
Hooray!