William Gilbert, an English physician and scientist, was not known for having a specific theory about why compasses work. He is famous for his groundbreaking work on magnetism and his book "De Magnete" (1600), where he proposed that the Earth itself is a giant magnet.

However, Gilbert's theory about Earth's magnetism did provide the foundation for understanding how compasses work.

Here's how:

* Earth as a magnet: Gilbert demonstrated that Earth behaves like a giant magnet, with a magnetic field extending around it.

* Magnetic poles: He also identified Earth's magnetic poles, which are points on the planet where the magnetic field lines converge.

* Compass alignment: He recognized that a compass needle aligns itself with the Earth's magnetic field, pointing towards the magnetic north pole.

Therefore, although Gilbert didn't explicitly state a "theory" about compass function, his work on Earth's magnetism provided the crucial knowledge to understand why compasses work.

It's important to note that Gilbert's theory was a significant advance in understanding magnetism, and it paved the way for future developments in the understanding of compasses and magnetic fields.