Spinoza's Ethics: II.P19

As Spinoza articulates the ability of the mind to understand the human body, he is also explaining how the human mind is subject to illusions. The human mind has direct connection with ideas of modifications of the body. This ideas are initially non-representative. In our parlance, we call these feelings. These modifications of the human body are true in themselves but we do not understand them in a cause effect way. This is, in our terms, subjective rather than objective knowledge. 

Insofar as God understand the human body, the idea is true and Spinoza describes how this is structured through the ideas of other singular things with clarity about causal connections. For us to understand the human body as God does, or even to move in that direction, means a better understanding of causal connections. While science provides some help, Spinoza's earlier propositions about God explore the critical role of necessity as manifesting true causal connections.

As a result, to make decisions with more reality or perfection or God - all of which are the same here - we can simply allow necessity to reveal itself as this force shows true causal connections. We can all identify those times where we called someone to just share a conversation or a recent event and found ourselves surprisingly hurt. Rather than making a call out of "necessity," we made the call "voluntarily." But this voluntary action was typically rooted in a subconscious desire for affirmation. When that subconscious desire was not met, we are suddenly disappointed. "Necessity" would have indicated two things: 1) don't make the call and 2) meet the subconscious desire by ourselves - leading to more power and an increase in perfection - which are the same here.

Mens humana ipsum humanum corpus non cognoscit nec ipsum existere scit nisi per ideas affectionum quibus corpus afficitur.

Translated as,

The human mind does not understand the human body itself and does not know that it exists unless by the ideas of modifications by which [the mind] is affected.

DEMONSTRATIO: Mens enim humana est ipsa idea sive cognitio corporis humani (per propositionem 13 hujus) quæ (per propositionem 9 hujus) in Deo quidem est quatenus alia rei singularis idea affectus consideratur vel quia (per postulatum 4) corpus humanum plurimis corporibus indiget a quibus continuo quasi regeneratur et ordo et connexio idearum idem est (per propositionem 7 hujus) ac ordo et connexio causarum, erit hæc idea in Deo quatenus plurimarum rerum singularium ideis affectus consideratur. Deus itaque ideam corporis humani habet sive corpus humanum cognoscit quatenus plurimis aliis ideis affectus est et non quatenus naturam humanæ mentis constituit hoc est (per corollarium propositionis 11 hujus) mens humana corpus humanum non cognoscit. At ideæ affectionum corporis in Deo sunt quatenus humanæ mentis naturam constituit sive mens humana easdem affectiones percipit (per propositionem 12 hujus) et consequenter (per propositionem 16 hujus) ipsum corpus humanum idque (per propositionem 17 hujus) ut actu existens; percipit ergo eatenus tantum mens humana ipsum humanum corpus. Q.E.D.

Translated as,

For the human mind is the idea itself or the understanding of the human body (by IIP13) which (by IIP9) is in God indeed insofar as another idea of a singular thing is considered an affect or because (by Postulate 4) the human body lacks many bodies by which continuously as if it is regenerated and the order and connection of ideas is the same (by IIP7) as the order and connection of causes, will be this idea in God insofar as it is considered an affect by the ideas of more singular things. Thus God has the idea of the human body or understands the human body insofar as [God] is an affect by some more ideas and not insofar as [God] constitutes the nature of the human mind that is 9by IIP11C) the human mind does not understand the human body. But the ideas of the modifications of the body are in God insofar as [God] constitutes the nature of the human mind or the human mind perceives those modifications (by IIP12) and consequently (by IIP16) the human body itself and it as actually existing; thus [God] perceives the human mind so long as the human body.