Adam Milligan's blog
I'm not sure a person can name a law after himself, but if I had a law I would want it to be this:
Any non-additive change to non-test code that causes no test failures is a valid change and does not reduce the correctness of the code.
I recently published the article There is no Agile, in which I stated that the principles of 'Agile' are nothing more than "a collection of good ideas, based on years of collective experience, for improving how we do our jobs." As an example, consider testing. Thorough testing is a ubiquitous principle of 'Agile;' thus, by logical extension, writing tests is a fundamentally important part of writing software well.
In short, if you write software, and you're not writing tests, then you're not doing your job.