Paradoxically, Spinozist eternity has a history: a mere extrinsic denomination in the Short Treatise, it becomes a real property in the Ethics and should be counted among the ranks of common notions. In this way, it stops belonging exclusively to substance to be extended to infinite and finite modes. The Ethics therefore marks a decisive turning point in relation to Metaphysical Thoughts, since Spinoza breaks the divine monopoly on eternity and replaces, once and for all, the doctrine of the immortality of the soul with that of the eternity of the intellect. At the same time, the status of modes finds itself shaken up: particular natura naturata is no longer content to enjoy a finite duration, or even an endless duration. It now sees itself endowed with an actual present existence and an actual eternal existence. The discovery of the exact ontological nature of eternity once again highlights the fecundity and originality of Spinoza's theory of common notions, for the existence of these infamous notions is the condition of possibility for the extension of a property characteristic of substance to infinite and finite modes. The theory of common notions thus picks up where the scholastic doctrine of communicable attributes had left off and turns out to be the key to reading the final propositions of the Ethics which establish the eternity of the human intellect. Without this theory, the communication of this property would appear to be a sleight of hand – more of a nominal than real communication. We can therefore understand why in Metaphysical Thoughts, where this doctrine is not yet elaborated, or at least does not explicitly appear, Spinoza indeed refrains from attributing eternity to created things and carefully reserves it for the creator, thereby avoiding the metaphysical conundrum of a ‘substantialisation’ of modes or a ‘modalisation’ of substance, as well as the accusation of creating a lesser divinity. Recourse to common notions allows us to evade these inextricable difficulties, for those properties which ground our reasoning express that through which two or more beings agree. They reveal the existence of a certain similarity between beings without implying an identity of nature. Substance and modes can therefore share eternity without for all that being of the same nature.