John Mackie (1917–81) noted that for almost every effect, there could be multiple causes at work. Suppose someone drops a cigarette in a house, which subsequently burns down. The house is unlikely to have burnt down just from the dropping of the cigarette. It also needed the presence of flammable materials, such as furniture, and plenty of oxygen so that the flame could take hold. The dropped cigarette was not sufficient on its own to cause the fire but it certainly was an essential part of the whole cause of it. In other words, there would not have been a fire without the cigarette.
Multiple causes
