All ordinary human knowledge works by comparison. To know something is to grasp its proportion to other things — its similarity and difference, its greater and lesser, its more and less. This mode of knowing is powerful within the domain of the finite, where proportions can be measured and compared. But God, as the absolute maximum, stands outside all proportional relations: God is not more or less than anything else, not similar or dissimilar to any other thing, not definable by any category that includes God in a class with other things. The human intellect can approach the infinite through a progressive series of comparisons and negations — an asymptotic approach — but it can never close the gap, because the gap between the finite and the infinite is not a large gap but an absolute one.
Cusa's learned ignorance goes beyond the negative theology of Pseudo-Dionysius and Maimonides, which holds that we can say what God is not (not finite, not spatial, not temporal) even though we cannot say what God is. For Cusa, even our negative propositions are problematic: to say "God is not finite" already implies a contrast with finitude that places God in a conceptual relationship with finite things. The learned ignorance is the recognition that not only our positive but also our negative predications of God are inadequate — that the divine coincidence of opposites means that God transcends both the affirmations and negations we direct at God.
The paradox of learned ignorance is that it is a genuine intellectual achievement, not a surrender of the intellect. The person who has worked through the whole range of philosophical and theological inquiry and arrived at docta ignorantia is wiser than the person who has not made the attempt — because they know the shape of what cannot be known, and this negative knowledge is more accurate and more honest than the pseudo-knowledge of those who think they have comprehended God within a concept or a system. For Cusa, Socratic ignorance is the beginning of wisdom, and the deepest wisdom is the recognition that the greatest object of inquiry infinitely exceeds the capacity of the inquiring mind.
Docta ignorantia is developed in De Docta Ignorantia (1440), Book I. Cusa states in the prologue that the concept came to him during a sea voyage from Constantinople in 1437, as a sudden illumination (divino munere) rather than through philosophical labour alone.