A scientist has a machine that simulates his brain and it is calibrated perfectly. He has a switch around his waist that can switch from controlling his body by his brains to by this machine. And though it shouldn't matter who is controlling the body, because they do exactly the same things and have the same thoughts, they both (brain and machine) yearn for the fact of knowing they are in control and not the other. And all that they have to know who is in control, is the knowledge of the position of the switch.
In an interview with a science journalist, where the scientist keeps insisting that it doesn't matter who is in control, as they both always have the same thoughts, the interviewer keeps taunting the him to turn the switch and let the other take control.
The subject keeps being very reluctant to turn the switch, but eventually does so, because the interviewer persuaded him. And then the scientist directly becomes overwhelmed with joy and gloats about finally being in charge of the body again.