When a person asks me a question, I've found it helpful to try to frame my response in the context of his own discipline or profession.
For example, when an attorney tells me he won't believe in the soul because it can't be measured physically, I ask him how can he prove in court that a non-physical thing like a motive exists if a motive can't be measured physically. Even laws themselves are not physical. They can't be weighed; they have no chemical composition; they aren't located in space. The attorney must face the fact that his own methodology defeats his objection.
This tactic makes it easier to persuade him, because he sees the issue in light of things he already knows to be true, or procedures he's already familiar with. This is an important step of establishing common ground.