Spinoza's Ethics: II.P30: Idea of the Duration of the Body

Here Spinoza addresses a fundamental concern for every person: knowing the extent of our lives. He does not diminish the quest here, but simply establishes that it is, so to speak, a closed book. The knowledge of an effect is dependent on its cause. Since the duration of our lives does not depend on our essence or on God, but only on an infinite chain of causality, knowledge of all things would be the requirement for an adequate understanding of the extent of our lives.

Nos de duratione nostri corporis nullam nisi admodum inadæquatam cognitionem habere possumus.

We are able to have nothing except a completely inadequate understanding about the duration of our body.

DEMONSTRATIO: Nostri corporis duratio ab ejus essentia non dependet (per axioma 1 hujus) nec etiam ab absoluta Dei natura (per propositionem 21 partis I). Sed (per propositionem 28 partis I) ad existendum et operandum determinatur a talibus causis quæ etiam ab aliis determinatæ sunt ad existendum et operandum certa ac determinata ratione et hæ iterum ab aliis et sic in infinitum. Nostri igitur corporis duratio a communi naturæ ordine et rerum constitutione pendet. Qua autem ratione constitutæ sint, ejus rei adæquata cognitio datur in Deo quatenus earum omnium ideas et non quatenus tantum humani corporis ideam habet (per corollarium propositionis 9 hujus); quare cognitio durationis nostri corporis est in Deo admodum inadæquata quatenus tantum naturam mentis humanæ constituere consideratur hoc est (per corollarium propositionis 11 hujus) hæc cognitio est in nostra mente admodum inadæquata. Q.E.D. 

The duration of our body is not dependent on its essence (by IIA1) and not even on the absolute nature of God (by IP21). But (by IP28) it is determined to exist and operate from such causes which are determined by others to exist and to operate by a clear and determined reason and these again from others and thus into infinity. Therefore, the duration of our body depends on the common order of nature and the constitution of things. Moreover, by which reason things are constituted, an adequate understanding of its thing is given in God insofar as ideas of all things and not insofar as it has such idea of the human body (by IIP9C); for what reason, the understanding of the duration of our body is in God is completely inadequate insofar as it is considered to constitute such nature of the human mind, that is (by IIP11C) this understanding is completely inadequate in our mind.