Are "soul" and "spirit" interchangeable?

A friend and I are having a discussion about whether to use Mind Body & Spirit or Mind Body & Soul for another friend’s new business.

I believe they have different connotations. She feels they are one & the same, but strongly favors the use of one over the other.

Do you feel there is a difference? Why or why not?

I’m not going into details of the direction the business is going because I don’t want it to influence the answers.