In this essay I analyse Kant’s view on the regulative role of reason, and in particular what he describes as the ‘indispensably necessary’ role of ideas qua foci imaginari in the Appendix. I review two influential readings of what has become known as the ‘transcendental illusion’ and I offer a novel reading that builds on some of the insights of these earlier readings. I argue that ideas of reason act as imaginary standpoints, which are indispensably necessary for scientific knowledge by making inter-conversational agreement possible. Thus, I characterise scientific knowledge as a distinctive kind of perspectival knowledge. This novel reading can illuminate the role of reason in complementing the faculty of understanding and sheds light on the apparent dichotomy between the first and the second part of the Appendix.More to the point, this novel reading takes us right to the heart of what knowledge is, according to Kant, and how it differs from bogus knowledge and opinion.