'Thou hast obeyed the Law. But mark well, when thou receivest good, so equally art bound to return good threefold.' (For this is the joke in witchcraft, the witch knows, though the initiate does not, that she will get three times what she gave, so she does not strike hard.)[High Magic's Aid by Gerald Gardner]
The Story, Glaucon, would take too long to tell; but the sum was this: --He said that for every wrong which they had done to any one they suffered tenfold .... for each and all of their offenses they received punishment ten times over, and the rewards of beneficence and justice and holiness were in the same proportion.
[The Republic by Plato, Book X, The Myth of Er]
Selena Fox, of Circle Sanctuary, wrote (here) that "Most Wiccans acknowledge that whatever magical force is sent out returns magnified to the sender." I have taken the liberty of emphasizing the word "magnified" in the quote, because I think that this puts the correct focus on what is truly important. The idea is not just that the results of of our actions return to us, but that my actions have more effect on me than they do on others. It is not just that when I harm others I am also harming myself - I am primarily harming myself and only incidentally harming anyone else. Agreeing on exactly what the "magnification" is (whether 3, or 10, or whatever) is far less important than understanding that this magnification does occur.
Wren Walker, of the Witches Voice website, has written (here) that "The Threefold Law takes the notion that 'what we reap, we will sow', a few steps further ... in fact, THREE steps further. For what we do 'for good or for ill, shall be returned to us threefold.' In light of this fact, Witches are loath to cause any harm..." This makes it clear that the Threefold Law is usually not just interpreted as applying to magic, but to all of our actions.
It is completely within the realm of the possible that Gardner was directly influenced by Plato's Myth of Er when he wrote about the Threefold Law of Return. References to Plato and other Greek philosophers and classical authors can be found scattered throughout High Magic's Aid, Witchcraft Today, and The Meaning of Witchcraft. However it is not necessary to claim such a direct influence merely to establish that the idea wasn't simply concocted by Gardner out of thin air.The idea of karma appears to have been already present in Western culture since even before Plato. In all probability the Greek concept of metempsychosis was taught by Pythagoras, and possibly even earlier by a philosopher named Pherecydes of Syros (whom some believe to have been one of Pythagoras' teachers). But it was Plato (who was born over 60 years after Pythagoras died) more than anyone else who popuarized the idea of metempsychosis, not only in the Republic, but also in his Gorgias, Phaedrus, Meno, Phaedo, Timaeus, and Laws.
For now it is important to separate out the idea of (a) any sort of spiritual progress being made from one lifetime to the next from (b) the more basic idea that the results (ie, "karma") of a person's actions carry over from one lifetime to the next. In fact there are four different ideas which, despite being closely related, need to be kept at least somewhat separate:
(1) that the results of all of our actions return to have an impact on us(2) that when our actions redound on us the impact is even greater than the impact on others
(3) that this process of returning karma persists from one lifetime to the next
(4) that the purpose of all this is for us to make spiritual progress over many lifetimes
It might appear that the Threefold Law only entails the first two of the above concepts. But we all know of examples of people who act badly throughout their lives and yet appear to suffer no (or very little) negative consequences, just as we also know of all too many cases of good people to whom bad things happen. Only if karma persists for more than a single lifetime is it plausible to insist that all of our actions redound on us, because without that it seems almost certain that it is possible to "cheat" karma. But as one American Buddhist teacher once quipped, "karma means you don't get away with nothin'."
But we needn't go all the way back to Plato to find indigenous (or at least indigenized) Western versions of the concepts of karma and reincarnation. German, French, British and American literature from the 18th, 19th and early 20th centuries offers many references to Seelenwanderung, palingenesis, metempsychosis, and reincarnation. And these ideas and terms are found in philosophy, poetry, drama, fiction, and even in political discourse! Two excellent books on this subject are: Secular Spirituality: Reincarnation and Spiritism in Nineteenth-Century France by Lynn L. Sharp, and Continued Existence, Reincarnation, and the Power of Sympathy in Classical Weimar by Lieselotte E. Kurth-Voigt.