Spinoza's Ethics: II.D5: Duration

Spinoza is beginning to establish an important aspect of existence of a mind or a body - its characterization by duration. Duration is not determined internally, because the nature of a mode is to go into and out of existence. Only substance whose essence is existence is not subject to duration.The continued existence of a mode is subject to a necessary causal chain - the same chain which brings a mode into existence also removes it from existence.

Spinoza is careful to highlight this as a condition of existing. Modes are subject to duration insofar as they exist. When modes do not exist, modes still an essence. As such, modes have an essence which is not subject to duration. When the essence of a mode begins to exist, the virtual becomes real. When that mode ceases to exist, then the real becomes virtual. 

Duratio est indefinita existendi continuatio.

Explicatio: Dico indefinitam quia per ipsam rei existentis naturam determinari nequaquam potest neque etiam a causa efficiente quæ scilicet rei existentiam necessario ponit, non autem tollit.

Translated as,

Duration is the indefinite continuation of existing.

Explanation: I say indefinite [continuation] because [the continuation] is not at all able to be determined by the nature of its own existence and moreover [the continuation] also does not come from the efficient cause which of course made the existence of the thing by necessity.