Iacobi Acontii Tridentini de methodo, hoc est, de recta investigandarum, tradendarumque artium, ac scientiarum ratione

Of those that act by necessity, some act by the effort of nature, others by external force. Those whose principle of acting is within themselves, with no external aid, act by the effort of nature, such as fire, to which an innate power of burning is given, and it does not cease to burn as long as there is something left into which it may unfold its power. Those are said to act by external force to which the power of acting has come from elsewhere, just as boiling water heats, not by its own power, but by the power of the fire from which it received heat. But the power of these is not lasting, but is remitted in such a way that they immediately return to their own nature. In those things that act by necessity, it is sufficient that they can act; in those, however, that act by choice, it is necessary that they also will.

There is also another distinction of efficient causes: for some are principal and commanding; some obey and serve, and of these, some act, and others are instruments: as in conducting a war, there are the commander, the soldier, and the weapons.

Furthermore, some causes are of such a kind that they alone suffice to produce an effect; others, however, must concur, as for the procreation of the male and female.