In some Neo-Pagan traditions, the belief that whatever you do will come back to you three times over. Thus, you should only do good things, as if you do something negative, that negativity will effect you much more strongly than what you did. A variation on the Tantric concept of karma.
Readers, please enjoy this guest blog post by Sandra Kynes, author of several books, including The Herb Gardener's Essential Guide, Plant Magic, and the new Witches' Encyclopedia of Magical Plants, among many others.
Although a few old books...