''Illusion is the kind of mirage that persists even though one knows that the ostensible object is not real' - Anthropology from a Pragmatic Point of View (1798), tr. Mary Gregor, The Hague, 1974, 53.
Within Dreams of a Spirit Seer, the parallel between ghostly visions and the
perception of optical illusions is made explicit in a passage that denounces
the seeing of apparitions as the delusion of an enthusiastic imagination. Kant does not explicitly introduce the term “projection,” which in German
becomes common around 1850 in referring to mental and optical processes.
But he describes how pathological spirit seers locate the figments of their
own imagination “outside of themselves,” mistaking these chimeras for the
actual presence of a specter. (Stefan Andriopoulos, 'Kant's Magic Lantern: Historical Epistemology and Media Archaeology', Representations , Vol. 115, No. 1 (Summer 2011), pp. 42-70: 48; Kant, Dreams of a Spirit Seer, Elucidated by Dreams of Metaphysics (1766), tr. D. Walford, in Immanuel
Kant, Theoretical Philosophy, 1755–1770 (Cambridge, 1992), 331.)
Kant held that human reason is beset with inescapable illusions that ground
classical metaphysics's arguments in favour of the immortality of the soul and
the existence of God, and generate endless debates about the nature of the
world (the totality of physical things). [Transcendental illusions: GT.] In the Transcendental Dialectic of the
Critique of Pure Reason, Kant undertakes to uncover the origin of what he calls
the illusions of reason, and to denounce the fallacies of the metaphysical arguments grounded on these illusions. One major diff1culty that has baffled com-
mentators is that Kant seems to maintain both that reason's illusions are
inevitable, and that 'special metaphysics' (inquiries into the existence and
nature of the soul, the world, and God) can and must be eradicated by the critical denunciation of its fallacious arguments. Is there a way out of this apparent inconsistency?
Michelle Grier argues that there is no inconsistency at all if one distinguishes, as Kant does, between the (inevitable) illusory representations of reason
and the (corrigible) fallacies of judgement by way of which those representations are referred to objects supposed to have actual existence and to be know-
able according to Kant's categories. Grier's merit is to follow up that distinction
systematically and thus to bring new light on the structure of Kant's complex
argument in the Transcendental Dialectic of the Critique of Pure Reason and its
Appendix, 'On the Regulative Employment of the Ideas of Pure Reason'. (Béatrice Longuenesse, 'Reviewed Work(s): Kant's Doctrine of Transcendental Illusion by Michelle Grier', Mind, New Series, Vol. 112, No. 448 (Oct., 2003), pp. 718-724: 718-9.)
§ 386. The two first parts of the doctrine of Mind embrace the finite mind. Mind is the infinite Idea; thus finitude here means the disproportion between the concept and the reality — but with the qualification that it is a shadow cast by the mind's own light—a show or illusion which the mind implicitly imposes as a barrier to itself, in order, by its removal, actually to realise and become conscious of freedom as its very being, i.e. to be fully manifested. The several steps of this activity, on each of which, with their semblance of being, it is the function of the finite mind to linger, and through which it has to pass, are steps in its liberation. In the full truth of that liberation is given the identification of the three stages — finding a world presupposed before us, generating a world as our own creation, and gaining freedom from it and in it. To the infinite form of this truth the show purifies itself till it becomes a consciousness of it. (The Philosophy of Mind, Encyclopaedia of the Philosophical Sciences (1817): http://www.gutenberg.org/files/39064/39064-h/39064-h.html.)
Fichte I must leave to others, of whom there are many on site, who possess the requisite knowledge.