A second ethical guideline is often called the Law of Return, sometimes the Rule of Three, which mirrors the physics concept described in Sir Isaac Newton's Third Law of Motion: "When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body." This basic law of physics is more usually today stated thus: "For every action, there is an equal and opposite reaction." Like the Rede, this guideline teaches Gardnerians that whatever energy or intention one puts out into the world, whether magical or not, will return to that person "multiplied by three." This teaching underlies the importance of doing no harm—for that would give impetus to a negative reaction centered on oneself or one's group (such as a coven). This law is controversial, as discussed by John Coughlin, author of "The Pagan Resource Guide," in an essay, "The Three-Fold Law."