Therefore all these names must be understood as common to deity as a whole, and as containing the notions of sameness and simplicity and indivisibility and union: while the names Father, Son and Spirit, and causeless and caused, and unbegotten and begotten, and procession contain the idea of separation: for these terms do not explain His essence, but the mutual relationship Greg. Naz., Orat. 45; cf. also Epist. ad. Evagr., and Greg. Nyss., Epist. ad Ablab.; Dionys., De div. nom., c. 2; Basil, Epist. 43 ad Greg. fratr. and manner of existence Dionys., De div. nom., c. 2; Greg. Naz., Orat. 37 and 45; Nyss. Epist. ad. Ablab..