For St. Basil, not the divine essence alone but also created essences could not be expressed in concepts. In contemplating any object we analyse its properties: it is this which enables us to form concepts. But this analysis can in no case exhaust the content of the object of perception. There will always remain an ‘irrational residue’ which escapes analysis and which cannot be expressed in concepts; it is the unknowable depth of things, that which constitutes their true, indefinable essence. In regard to the names which we apply to God, these reveal his energies which descend towards us yet do not draw us closer to his essence, which is inaccessible.
Vladimir Lossky, The Mystical Theology of the Eastern Church (Crestwood, New York: St. Vladimir’s Seminary Press, 1997), 33.