I experience intellectual work, such as testing, as a web of interconnected activities. If I were to suggest what is at the center of the testing web, on my short list would be: designing experiments. A good test is, ultimately, an experiment.
I’ve been looking around online for some good references about how to design experiments (since most testers I talk to have a lot of trouble with it). Here is a good one.
If you know of any other straightforward description of the logic of experiments, please let me know. I have some good books. I just need more online material.