In the Architectonic of Pure Reason, Kant claims that a condition for attaining the status of science is that a body of cognitions achieves architectonic unity. The chapter clarifies what this is and argues that this is more than mere systematicity. While there could be different ways of systematically ordering a body of cognitions, there is only one such order that bestows architectonic unity to it. Architectonic unity is achieved when the ‘idea’ of a science given a priori by reason is realized in an actual body of cognitions. I read this ‘idea’ as the correct description of the body of cognitions that form a science and its parts–whole relationships. In a further step, I discuss the ‘idea’ that is a candidate for providing unity to metaphysics. Kant identifies two candidate ‘ideas’ according to which metaphysics can be understood. Metaphysics can be construed according to either the ‘school concept’ (Schulbegriff) or the ‘worldly concept’ (Weltbegriff) of philosophy. I argue that it is only according to the ‘worldly concept’ that metaphysics can attain architectonic unity and become a science.