E3P54D
Demonstration — Part III
Latin
Mentis conatus sive potentia est ipsa ipsius mentis essentia (per propositionem 7 hujus); mentis autem essentia (ut per se notum) id tantum quod mens est et potest, affirmat; at non id quod non est neque potest adeoque id tantum imaginari conatur quod ipsius agendi potentiam affirmat sive ponit. Q.E.D.
English (Elwes 1883)
The endeavour or power of the mind is the actual essence thereof (III. vii.); but the essence of the mind obviously only affirms that which the mind is and can do; not that which it neither is nor can do; therefore the mind endeavours to conceive only such things as assert or affirm its power of activity. Q.E.D.