P14: If the Mind has once been affected by two affects at once, then afterwards, when it is affected by one of them, it will also be affected by the other.

Dem.: If the human Body has once been affected by two bodies at once, then afterwards, when the Mind imagines one of them, it will immediately recollect the other also (by IIP18). But the imaginations of the Mind indicate the affects of our Body more than the nature of external bodies (by IIP16 C2). Therefore, if the Body, and consequently the Mind (see D3) has once been affected by two affects {NS: at once}, then afterwards, when it is affected by one of them, it will also be affected by the other, q.e.d.