This is something I have seen in one of the testing related online trainings:

 Testing is the unregulated art of evaluating the invisible against the ambiguous so as to avoid the unthinkable.

 Why is it an unregulated art?

I think it is an art because nobody can teach testing. One can teach testing methodologies process but at the end you need to tester’s smartness to unearth a bug.

 What is evaluating the invisible?

When you get a hunch that there is might be a bug, you write repro code, try out user action many times to evaluate your guess

 What is avoid the unthinkable?

Unthinkable #1: Customer hitting the bug which you couldn’t find

Unthinkable #2: Being fired because you shipped product with a showstopper? J