Kant substitutes, for the traditional concept of error (error as product in the mind of an external determinism), that of false problems and internal illusions, These illusions are said to be inevitable and even to result from the nature of reason. All Critique can do is to exorcise the effects of illusion on knowledge itself, but it cannot prevent its formation in the faculty of knowledge.

(Gilles Deleuze "Kant's Critical Philosophy")

Thus the white lily is not merely related to the concepts of colour and flower, but also awakens the Idea of pure innocence, whose object is merely a (reflexive) analogue of the white in the lily flower. We can see here how the Ideas are the object of an indirect presentation in the free materials of nature. This indirect presentation is called symbolism, and has as its rule the interest of the beautiful.

Two consequences follow from this: the understanding itself sees its concepts enlarged in an unlimited way; the imagination is freed from the constraint of the understanding to which it remained subject in the schematism and becomes capable of reflecting from freely. The accord between imagination as free and understanding as indeterminate is therefore not merely assumed: it is in a sense animated, enlivened, engendered by the interest of the beautiful.

(Gilles Deleuze "Kant's Critical Philosophy")