To E. A. Bennet
Dear Bennet, 11 June 1960
Thank you very much for your illuminating letter.
I see from it that you understand by “scientific evidence” something like a chemical or physical proof.
But what about evidence in a Law Court?
The concept of scientific proof is hardly applicable there, and yet the Court knows of evidence which suffices to cut a man’s head off, which means a good deal more than the mere universality of a symbol.
I think that there is such a thing as “commensurability of evidence.”
Obviously the way of proving a fact is not the same and cannot be the same in the different branches of knowledge.
For instance, the mathematical method is not applicable either in psychology or in philosophy and vice versa.
The question ought to be formulated: what is physical, biological, psychological, legal, and philosophical evidence?
By which principle could one show that physical evidence is superior to any other evidence?
Or how could anybody say that there is no psychological evidence for the existence of a quantum or a proton?
Obviously no branch of knowledge can be expressed in terms of another branch, just as one cannot measure weight by kilometres or length by litres or ohms by volts .
There is also no “scientific proof” of the existence of the migration instinct, for instance, yet nobody doubts it.
It would be too much to expect chemical proof in a murder case, yet the case can be proved by a legal method quite satisfactorily.
Why should psychology be measured against physics-if one is not a member of the Leningrad Academy?
C.G. Jung ~Carl Jung, Letters Vol. II, Pages 565-566