The Kantian conception of aesthetic idea consists of three notions. First, there is a rational idea, which is an abstract notion, typically of moral significance that is not a part of our experience, such as eternity or creation, and whose full significance is not part of our experience, such as death or love. This rational idea can be regarded as the content of the work of art at the most abstract level. Second, there is overarching image of the imagination through which the abstract idea is presented—Kant offers the example of Jupiter as a symbol of divine power and justice. Third, there is what Kant calls the wealth of “aesthetic attributes” suggested by unrestricted reflection on the image of the imagination—he talks about “Jupiter’s eagle with the lightning-bolts in its claws…” Kant distinguishes the “aesthetic attributes” from the “logical attributes” contained in the already given content of the idea.
Here’s an excerpt from Kant’s the Critique of Judgement where he is talking about the aesthetic attributes in Jupiter’s eagle:
Those forms which do not constitute the presentation of a given concept itself but only, as approximate representations of the Imagination, express the consequences bound up with it and its relationship to other concepts, are called aesthetic attributes of an object, whose concept as a rational Idea cannot be adequately presented. Thus Jupiter’s eagle with the lightning-bolts in its claws is an attribute of the mighty king of heaven, as the peacock is of its magnificent queen. They do not, like logical attributes, represent what lies in our concepts of the sublimity and majesty of creation, but something different, which gives occasion to the Imagination to spread itself over a number of kindred representations, that arouse more thought than can be expressed in a concept determined by words. They furnish an aesthetic Idea, which for that rational Idea takes the place of logical presentation; and thus as their proper office they enliven the mind by opening out to it the prospect into an illimitable field of kindred representations. But beautiful art does this not only in the case of painting or sculpture (in which the term “attribute” is commonly employed): poetry and rhetoric also get the spirit that animates their works simply from the aesthetic attributes of the object, which accompany the logical and stimulate the Imagination, so that it thinks more by their aid, although in an undeveloped way, than could be comprehended in a concept and therefore in a definite form of words.Kant goes on to make it clear that the notion of the aesthetic idea is complex, because of the three elements being involved in the artwork that is exhibiting an aesthetic idea. “In a word the aesthetic Idea is a representation of the Imagination associated with a given concept, which is bound up with such a multiplicity of partial representations in its free employment, that for it no expression marking a definite concept can be found; and such a representation, therefore, adds to a concept much ineffable thought, the feeling of which quickens the cognitive faculties, and with language, which is the mere letter, binds up spirit also.”