Quanta Magazine: Untangling Why Knots Are Important

Adams (23:51): Yeah, so slice comes back to this fact that I mentioned which is every knot in four-space is trivial. In other words, every knot in four-space can be deformed to the trivial knot, and once you deform it to the trivial knot, you could actually deform it into a plane, a two-dimensional plane in four-space. And that two-dimensional plane bounds a disk.