Spinoza's Ethics: II.P16

Idea cujuscunque modi quo corpus humanum a corporibus externis afficitur, involvere debet naturam corporis humani et simul naturam corporis externi.

Translated as,

An idea whose mode in which the human body is affected by external bodies ought to involve the nature of the human body and at the same time the nature of the external body.

DEMONSTRATIO: Omnes enim modi quibus corpus aliquod afficitur ex natura corporis affecti et simul ex natura corporis afficientis sequuntur (per axioma 1 post corollarium lemmatis 3) : quare eorum idea (per axioma 4 partis I) utriusque corporis naturam necessario involvet adeoque idea cujuscunque modi quo corpus humanum a corpore externo afficitur, corporis humani et corporis externi naturam involvit. Q.E.D.

Translated as,

For all modes by which some body is affect follow from the nature of the affected body and at the same time from the nature of the affecting body (by A1 after L3C): wherefore the idea of those (by IA4) involves by necessity the nature of each body to the extent that the idea of whose mode in which the human body is affected by an external body involves the nature of the human body and the of the external body.

COROLLARIUM I: Hinc sequitur primo mentem humanam plurimorum corporum naturam una cum sui corporis natura percipere.

Translated as,

From here it follows first that the human mind perceives the nature of more bodies with the nature of its own body.

COROLLARIUM II: Sequitur secundo quod ideæ quas corporum externorum habemus, magis nostri corporis constitutionem quam corporum externorum naturam indicant; quod in appendice partis primæ multis exemplis explicui.

Translated as,

Next it follows that the ideas which we have of external bodies indicate more the constitution of our own body than the nature of the external bodies; which I have explained in the appendix of part one by means of multiple examples.