unalterable (adj.) Look up unalterable at Dictionary.com
1610s, from un- (1) "not" + alterable. Related: Unalterably.
He reach'd a middle height, and at the stars,
Which are the brain of heaven, he look'd, and sank.
Around the ancient track march'd, rank on rank,
The army of unalterable law.
[George Meredith, "Lucifer in Starlight"]