  It has long been recognized as a difficult problem to determine whether the
observed statistical correlation between two classical variables arise from
causality or from common causes. Recent research has shown that in quantum
theoretical framework, the mechanisms of entanglement and quantum coherence
provide an advantage in tackling this problem. In some particular cases,
quantum common causes and quantum causality can be effectively distinguished
using observations only. However, these solutions do not apply to all cases.
There still exist enormous cases in which quantum common causes and quantum
causality can not be distinguished. In this paper, along the line of
considering unitary transformation as causality in the quantum world, we
formally show quantum common causes and quantum causality are universally
separable. Based on the analysis, we further provide a general method to
discriminate the two.
