[atburrow]

Isn't your "Language comprises rules that change over time." statement invalid as well? The word comprises should be used when there exists a definite set of items.

For example: "That house comprises 4 rooms" versus "That house comprises at least 4 rooms". The second statement should be "That house is composed of at least 4 rooms" because the room count is not an exact quantity.

[rayiner]

I think you're right. In my head I was equating "language" with "grammar" and I think it's fair to say that you can specify grammar with a definite set of rules. But at least human language has more than that.