The Law of Return
Also known as the "Threefold Law" or the "Rule of Three"
There are many variations of the phrasing of the Threefold Law, but it generally goes something like:
Ever Mind The Rule Of Three
Three Times Your Acts Return To Thee
This Lesson Well, Thou Must Learn
Thou Only Gets What Thee Dost Earn1
When you take a resource, even with good intentions, there will be repercussions. The petty cash box will soon be empty if one doesn't occasionally put money into it as well as take it out. Actions need to be balanced, or else things can get quite out of control to a degree many times over that of the original issue.
In other words, there's no such thing as a free lunch.
Other phrasings emphasize completely separate issues, which author Phyllis Curott has recently taken to calling the Boomerang Whammy Rule:2
Mind the Threefold Law you should,
Three times bad and three times good.
The above version was taken directly from the Credo, a piece of work that I do not believe is required reading for Wiccans but is interesting to investigate at the least. Or, for an even more extreme version:
Ensure that your actions are honorable,
for all that you do shall return to you, threefold, good or bane.
People attempt to pass this phrasing off as a moral code, which it is not. The Threefold Law is a statement of belief in the ways of the universe. It does not teach us what is "bad" or "good", only that we shall receive three times whatever we give. The only reason it offers for being good is to receive reward and to escape punishment. That is not morality.
The world does not work as simply as these phrases make it sound. If it did we'd all be donating to charity like mad and reaping the rewards by the handful. The idea of things returning threefold is unnatural. According to the Law of Ecology (from biology class - as Wiccans we should be taking lessons from nature):
Everything is connected to everything else
Everything must go somewhere
Nature knows best
There is no such thing as a free lunch
But it is true that harm tends to beget harm, and it is true that one good turns deserves another: people remember a person's charity and are more likely to aid them in return. Hence, why I prefer to use the term "Law of Return" over "Threefold Law".
Let's also remember one of Newton's laws as another lesson from nature: for every action there is an equal and opposite reaction. "Opposite" does not mean that you receive bad for every good. It means what gets put out comes back. For instance, if you push upon a wall, the wall is actually pushing back with an equal amount of force - if it did not, it would fall over. That's straight from physics class.
However, what counts as "equal" is not always obvious, especially when dealing with magic, which is what the Law of Return largely addresses. You are invoking the gods for a favor. Therefore, there is a sort of "tax" involved, and this tax is dependent upon the nature of the magic being worked. "Good" should really be thought of as "in harmony with nature", while "bane" is not evil, but instead working contrary to natural law. The more baneful a request, the more resistant the world will be to your intended change. For instance, a ritual asking for a healthy mother gives birth to a healthy child is fairly straightforward, while one asking for a cocaine-addicted mother to give birth to a healthy child is going to take considerably greater effort, even though the desired result (a healthy child) is the same. (See more in Magic)
"The Threefold Rule follows the old laws of karma"
First of all, there is nothing in the rule of karma involving threes. Second, karma is "the force generated by a person's actions, believed in Hinduism and Buddhism to determine his/her destiny in the next existence."3 Karma determines what happens in your next life, not next week.
And if you ever claim that the Threefold Rule is both karmic and Celtic, I swear I will personally kick your ass on general principle.