Transcendental unity is a figure of thought of the Latin Middle Ages, which is indebted to Avicenna’s renewal of metaphysics and which is wrongly attributed to Aristotle. A specific interpretation of the demonstrable attribute determines the metaphysical reflection on ‘the one’ and turns it into a transcendental attribute of being. Notwithstanding the variety of epistemic constellations, however, this metaphysical relationship of being and unity always turns out to be a fundamental state of affairs. Transcendental unity identifies as a problem constellation, the principles of which are still effective in the critique of scholastic metaphysics in classical German philosophy.