Emptiness and loss of meaning are expressions of the threat of nonbeing to the spiritual life. This threat is implied in man’s finitude and actualized by man’s estrangement. I can be described in terms of doubt, its creative and its destructive function in man’s spiritual life. Man is able to ask because his is separated from, while participating in, what he is asking about. In every question an element of doubt, the awareness of not having, is implied. In systematic questioning systematic doubt is effective; e.g. of the Cartesian type. This element of doubt is a condition of all spiritual life. The threat to spiritual life is not doubt as an element but the total doubt. If the awareness of not having has swallowed the awareness of having, doubt has ceased to be methodological asking and has become existential despair.
~Courage to Be, Tillich, 48
