Spinoza's Ethics: II.P25: Our Limit to Understanding What Affects Us

Spinoza develops a proposition here that elucidates just how little we have an adequate understanding of that which affects us. His proof involves a causality - because something existed in nature prior to affecting us, this nature is understood in God in a way that is unrelated or as Spinoza puts it, uninvolved with our understanding. In a practical sense, when something affects us, the full understanding of the affect of that thing must be understood from its nature not from the impact we experience and thus impute. This framework is useful in reducing our tendency to personalize experiences and others.

Idea cujuscunque affectionis corporis humani adæquatam corporis externi cognitionem non involvit.

Translated as,

The idea of whatever modification of the human body does not involve the adequate understanding of the external body.

DEMONSTRATIO: Ideam affectionis corporis humani eatenus corporis externi naturam involvere ostendimus (vide propositionem 16 hujus) quatenus externum ipsum humanum corpus certo quodam modo determinat. At quatenus externum corpus individuum est quod ad corpus humanum non refertur, ejus idea sive cognitio in Deo est (per propositionem 9 hujus) quatenus Deus affectus consideratur alterius rei idea quæ (per propositionem 7 hujus) ipso corpore externo prior est natura. Quare corporis externi adæquata cognitio in Deo non est quatenus ideam affectionis humani corporis habet sive idea affectionis corporis humani adæquatam corporis externi cognitionem non involvit. Q.E.D.

Translated as,

We have shown (See IIP16) that the idea of the modification of the human body insofar as it involves the nature of an external body to such an extent the external body certainly determines the human body itself in a certain way. But insofar as the external body is an individual which does not refer to the human body whose idea or understanding is in God (by IIP9) insofar as God is considered an affect of another thing, the idea which (by IIP7) is prior to the external body itself in nature. Thus, the adequate understanding of the external body is not in God insofar as it has the idea of the modification or the human body or the idea of the change of the human body does not involve the adequate understanding of the external body.