P12: Whatever happens in the object of the idea constituting the human Mind must be perceived by the human Mind, or there will necessarily be an idea of that thing in the Mind; i.e., if the object of the idea constituting a human Mind is a body, nothing can happen in that body which is not perceived by the Mind.

Dem.: For whatever happens in the object of any idea, the knowledge of that thing is necessarily in God/Nature (by P9 C), insofar as he is considered to be affected by the idea of the same object, i.e. (by P11), insofar as he constitutes the mind of some thing. Therefore, whatever happens in the object of the idea constituting the human Mind, the knowledge of it is necessarily in God/Nature insofar as he constitutes the nature of the human Mind, i.e. (by P11 C), knowledge of this thing will necessarily be in the Mind, or the Mind will perceive it, q.e.d.

Schol.: This Proposition is also evident, and more clearly understood from P7 S, which you should consult.