Typically there’s more than one cause for an effect. Even if one causal factor seems to have contributed in particular to a certain outcome, other factors were almost certainly involved. Often we tend to forget this when we look for causes, and end up focusing too narrowly on what might only be a triggering factor.
Say a fire started because a smoker fell asleep in bed with a cigarette still burning in her hand. Certainly the cigarette would have triggered the fire, but it couldn’t have done so in isolation. A burning cigarette can do nothing unless it interacts with something that itself can burn, a piece of cloth or furniture, for instance. There must also be some oxygen or other flammable gas, otherwise there would be no fire.
So if we have the burning cigarette, the cloth and some oxygen, the fire might develop. But only if the cloth is dry and there’s enough oxygen to sustain the fire. Now, the fire might run its course until the cloth is entirely burned. Or it could escalate, if the burning cloth is near something else that could catch fire, a bed, a carpet, a wooden floor, and so on.
We could call the fire, the flammable material and the oxygen mutual manifestation partners. What qualifies as manifestation partners is that they can do something together than none of the partners could have done alone. Only together with such manifestation partners can a cause produce an effect.
Why is it important to acknowledge that a cause never acts alone? Basically, it’s because we could end up overlooking other significant causal contributors.
A gene might causally contribute to a trait, but without the organism in its environment, the gene would be causally inert. Similarly, a pill might cure a headache, but only through interaction with certain biochemical processes in a body. By focusing only on one factor, perhaps the last that was added, we might conclude that the other parts were irrelevant. But the straw that broke the camel’s back is not the full causal story. We must also account for the other straws.