Mind and matter are two sides of the same coin. This is at least the intuition that two of the most brilliant minds of the 20th century on Theoretical Physics and Psychology, respectively, shared and tried to grapple with collaboratively. This collection of letters is the result of them trying to make the parallels at the depths of their fields mutually intelligible and work out their implications. The fundamental double parallel ("quaternio" if you will) looks like this:
Archetype : Energy (both atemporal) :: Self-Consciousness : Sequence of Physical Events (both temporal)
In plain language, this means that there are certain unchanging qualities, archetypes and energy, which seem to share an existence outside of time (and therefore conscious experience as well as physical events). In other words, observation (in the physical as well as psychological sense) cannot happen from a viewpoint of nowhere / -when. In this respect, the apparently incongruent pieces of mind and matter can conjoin to create a whole in their complimentarity.
As a result of 26 years of sending letters back and forth, countless puzzling dreams reports interspersed with mythology and metaphysical speculation, Pauli and Jung are able to piece together this complementarity of viewpoints they share. This book is a fascinating account of how two lucid minds try their best in equating the different languages through which their minds express the apparently same (or at least similar) ideas about reality. At times, the regular reference to esoteric ideas & thinkers can be estranging, though. It entangles the lucid train of thought into a web rather bewildering treatises of unheard-of mystic thought. However, these two men seem to have known what they were talking about. After all, these letters were not written to be published. We should be glad they were published nevertheless, despite the more outlandish passages certainly confusing to everyone but the hardcore mystic.