In this article, we examine three concepts—eternity, duration, and time—in Spinoza’s philosophy, particularly in his Ethics. Contrary to most prominent interpretations, the concept of eternity cannot be explained, either negatively or positively, in relation to duration or time. Referring to Spinoza’s own texts, we argue that, according to the criterion he sets for definitions, the definition of anything necessarily involves its essence and cause. Eternity, as an existence that follows from the definition itself, is therefore necessary, since both the cause and the essence necessarily affirm the existence of the thing. Eternity cannot be explained through duration, because durational existence—unlike eternal existence—is determined by factors external to the definition of the thing. Duration is the continuity of a thing’s existence, and every finite being, by virtue of its essence, demands an unlimited duration. Yet, since the cause of a finite thing is always another finite thing, this duration cannot be determined through the definition of the thing itself and is constantly threatened by other finite beings. Nevertheless, duration can be understood in two ways: on the one hand, as the unlimited and continuous persistence of existence; and on the other, in an abstract sense, as a particular kind of quantity. Time is nothing but duration, insofar as it is viewed through imagination and from a finite standpoint, that is, when it has become determined as a kind of quantity.