The Law of Conservation of Pain was originally stated in "Laws of Conservation of Pain and Joy" in Callahan's Crosstime Saloon and stated "Just as there are Laws of Conservation of Matter and Energy, so there are in fact Laws of Conservation of Pain and Joy. Neither can ever be created or destroyed. But one can be converted into the other."
The shortened version, which appeared in the afterword of Callahan and Company stated "Shared pain is lessened; shared joy, increased — thus do we refute entropy." and was called Callahan's Law.