To translate without betrayal, it is necessary to first determine whether eternity is a category, a genus divisible into species, a form capable of assuming several appearances (aspects), or that ‘je ne sais quoi’ which differentiates between the finite and the infinite. An elucidation of its nature must serve as a prolegomenon to any interpretation of the phrase sub specie aeternitatis and entails, above all, the definition of its place in Spinozist ontology – an ontology that allows for only three well-known divisions between substance, attributes and modes. Should we then consider eternity at the level of a substance, an attribute or a mode? A slightly informed reader will immediately rule out the first hypothesis since there is only one unique substance, called God; such a reader will be equally tempted to immediately push back on the second by arguing that we are only familiar with, by Spinoza's own admission, two attributes – namely, thought and extension. If eternity figured among the infinity of attributes, Spinoza would have in all likelihood taken care to emphasise such a point. The author of the Ethics mentions nothing of the sort and limits himself to alluding to other attributes without providing any further details. Metaphysical Thoughts, however, causes such an immediate certainty to waver, for the first chapter of Part II opens with a disconcerting claim according to which ‘the chief attribute, which deserves consideration before all others, is God's Eternity …’. Is this an ambiguity in vocabulary? A terminological mutation? A development in Spinoza's thought? Such are among many questions that only a careful examination can permit us to resolve.

The third hypothesis seems to better accommodate us in the lodge of evidence, but when it comes down to it, its accommodations are hardly comfortable. If eternity is a mode, is it so in the same way as singular finite things, as the divine intellect or the facies totius universi? As real as eternity is, it is obvious that it does not express substance in the same way as does Peter, Paul or Simon. In what sense can eternity be a mode? Eternity, like duration, cannot be reduced to a mere mode of thought, and is thereby distinguished from time, to which Spinoza confers this ontological status.