The theory that all knowledge is derived from sense-experience (i.e. from experimentation).
When empiricism was first introduced it played a positive role for Progress by providing a defence against traditional authorities and dogma. All claims of the supernatural now had to be backed up by hard evidence, which meant that 'provable' science superseded religious mysticism and superstition.
This worked for a while because none of the scientists took it literally. Because, if they had, they wouldn't have made much progress.
The reason that testability is not sufficient is that prediction is not, and cannot be, the purpose of science. You can test that a rabbit comes out of a hat during a conjuring trick, and predict that it'll happen every time the trick is performed, but that gets you no closer to understanding how the trick works.
In other words strictly applied empiricism is a threat to Progress and indeed has held many fields back, such as quantum physics.
The real source of our theories is Conjecture, and the real source of our Knowledge is conjecture alternating with Criticism. We create theories by rearranging, combining, altering and adding to existing ideas with the intention of improving upon them. The role of experiment and observation is to choose between existing theories, not to be the source of new ones.