Russian émigré theologian Vladimir Lossky (1903-1958) claims that Western theologies of the dark night of the soul tend toward an impersonal philosophy, especially neo-Platonism, in a way that seems to involve spiritual dangers. At the same time he commends, for example, the Orthodox theologies of St Symeon the New Theologian (949-1022) and St Maximus Confessor (580-662). This study discusses the Spanish Carmelite St John of the Cross’ (1542-1591) mystical theology of dark nights of the soul from the point of view of Lossky’s claim, conducting a substantial comparison of St John’s theology with the theology of St Symeon the New Theologian. In addition, it also compares select aspects of his theology with aspects of Vladimir Lossky’s and St Maximus Confessor’s theologies and the thought of the neo-Platonist philosopher Plotinus (204-270). The purpose of these comparisons is to propose a definition of the relationship of St John of the Cross’ theology to central Orthodox theological principles and emphases and to evaluate the truth of Lossky’s more general attempt to define the Western notion of dark nights from an Eastern Orthodox perspective.