Organon §21

A medicine's power to cure is revealed in the symptoms it engenders in healthy people. These symptoms reveal the medicine's particular disease-engendering power which is the same as its disease-curing power.

1. the curative wesen in medicines is not, in itself, discernible (which no one can deny), and

2. in pure experiments conducted with medicines [i.e. , in provings], the most sharp-witted observer can perceive nothing about medicines that can constitute them as medicines, or curative means, except their power to bring forth distinct alterations in the condition of the human body, and especially their power to differently tune the healthy person in his condition and to arouse several particular disease symptoms in him,

then it follows that when medicines act as curative means, they likewise can only bring their curative capacity into execution through this, their power to differently tune the human condition by means of engendering peculiar symptoms. Therefore, we only have to abide by [act in conformity with] the disease befallments that medicines engender in the healthy body as the only possible revelation of their in-dwelling curative power. From these disease befallments, we experience the disease-engendering power which is, at the same time, the disease-curing power that each single medicine possesses.