To H . Rossteutscher
Dear Herr Rossteutscher, 20 May 1958
My remarks about the concept of "reciprocal effect" as a causalistic premise naturally do not imply that the reciprocal effect and/or causality is absolutely out of the question in the realm of parapsychological phenomena.
The explanation depends entirely on the experience itself.
It is altogether possible that cases which we today explain as synchronistic will tomorrow turn out to be causal in a manner that cannot yet be foreseen.
I must confess, however, that I wouldn't expect such a surprise in the case of precognition.
The classic example of reciprocal effect is surely complementarity.
Should it turn out that the majority of synchronistic phenomena are by their very nature reciprocal effects, then the term synchronicity would become manifestly superfluous.
But this proof has still not been furnished at present and, as I have said, in the case of precognition it is even more improbable.
On the other hand, we know with sufficient certainty that the so-called causal law is a statistical truth, and accordingly there must be exceptions for which a causal explanation is, shall we say, valid for only 40% of them, the remaining 6o% coming into the category of acausality, just like precognitive phenomena.
Obviously there would be no parapsychological phenomena whose very nature presupposes a reciprocal effect if such an effect were to be ruled out from the start.
But at present we have-and this is my premise-no sufficient empirical grounds for assuming that parapsychological phenomena are in all cases based on reciprocal effect.
This is not to say that there is no reciprocal effect whatever, but merely that this is not the decisive factor.
Here you have the justification for my hypothesis of a principle which is based not on causality but on the equivalence of the meaning of events.
This whole question should be thoroughly immunized against philosophical speculations.
Experience alone can help us forward.
C.G. Jung ~Carl Jung, Letters Vol. II, Pages 436-437