that identity that he calls the idea are as follows. First of all, these ideas or εἶδη, are ἀίδια, i.e., eternal and ever the same, without change, in contrast to things that change. He explains that by saying they are ἀγένητα καὶ ἀνώλεθρα, i.e., what he calls an idea does not come into being or pass away, and is indestructible, free of change. The distinction between real and ideal being goes back to this basic ontological distinction in Greek philosophy, back to Plato.

Now we may characterize more precisely these two regions of being—the abiding, self-same ideal versus the changing real—in terms of the way we apprehend them. What is permanent about a sensible object is the element that is apprehended by [intuitive] reason, νοῦς. This means that the ideas or the idea is the νοητόν, the knowable. By contrast, the multiple things of the real world are accessible to sensibility—αἴσθησις—and so each is designated as the αἰσθητόν. Here again, ideal and real being are characterized in terms of the specific mode of access that we have to them, and not in terms of their being or modality of being. What is more, this distinction has continued up to our own day and above all, plays a major role in Kant’s philosophy. So for example, a book of Kant’s, his so-called dissertation of 1770 (which in fact is the real prelude to his Critique of Pure Reason), is titled De mundi sensibilis atque intelligibilis forma et principiis.³² The intelligibile corresponds to the νοητόν, and the sensibile corresponds to the αἰσθητόν. Kant speaks of an intelligible and [58] a sensible world, and today as well we speak of sensible and non-sensible being and mean this distinction of the real and the ideal. This distinction still prevails today, and it dominates all inquiries, ontological as well as epistemological.

In refuting psychologism and, positively, in founding logic, there is a fundamental ontological difference that provides the proper orientation. To summarize a bit, the ideal is what always is. It is the permanent as against its changing instantiations. It stands in contrast to those ways in which it shows itself in any given instance and comes into appearance—i.e., in contrast to how it [sensibly] appears. The ideal is likewise the universal in contrast to the multiplicity of its individual instances. The idea “triangle” is the universal that is instantiated in each particular triangle. This concept of the ideal, specifically in its triple meaning of the self-identical, the permanent, and the universal, is the guiding thread of Husserl’s critique of psychologism. At the same time it is the guiding thread for determining the being of truth

32. [In Akademie-Ausgabe, vol. 2, ed. Erich Adickes (Berlin: Walter de Gruyter, 1905; reissued, 1968), pp. 385–420; see also “On the Forms and Principles of the Sensible and the Intelligible World (1770),” in Immanuel Kant, Theoretical Philosophy, 1755–1770, trans. and ed. David Walford, with Ralf Meerbote (Cambridge: Cambridge University Press, 1992), pp. 373–416.]