First of all, I do not believe this is literally or strictly the case; it is more of a thought experiment

Any person can be bought or induced to act based on external intentions; they can be "purchased" in some way.

If we had complete knowledge of everything involving that person's brain, we would know exactly how to buy them.

It might involve reinforcing their ideology, using drugs and altering their mental state, offering money, power, social status, a sense of belonging, religious validation,...