if the causal principle is only relatively valid, then it follows that even though in the vast majority of cases an apparently chance series can be causally explained, there must still remain a number of cases which do not show any causal connection.
Jung argues that a merely relative causal principle logically necessitates the existence of genuinely acausal chance events, establishing the theoretical ground for synchronicity.
, The Structure and Dynamics of the Psyche, 1960thesis