What is an "efficient cause"?
Lets start with the phrase "efficient cause". What is efficient about it? The idea comes from Aristotle and his "four causes". He distinguished between different kinds of causes of a thing. A building is caused by the blue print, by the wood and nails, and by the need of a family. It is also caused by a team that builds it. Those people are what Aristotle called the efficient cause of the house.
We might say that the efficient cause is the actor; in most causes we might think of, its the humans who causes the action.
A statue is caused by a model, a medium, and a sponsor; it is also caused by the sculptor placing the chisel. (See the diagram) The sculptor is the efficient cause.
Prof. John Lennox often gives the example of both the laws of internal combustion and Henry Ford as explanations of the motor car. Ford here is the efficient cause.
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I know that's a lot of examples. That's the point: there are many examples of efficient causes. In fact they are literally everywhere. Everything we experience has efficient cause--something that acted on it. We understand this instinctively.
The last step in this arguement is to recognize that efficient causes cannot go back infintely. For me, this makes most sense when I try to think forward from cause to effect, not backward. Imagine you are waiting for a train. This train is still on its way, andf it has to make an infinite number of stops before it gets to you. In other words, there is always one more stop before you. Will you ever get on the train? No.
That first cause cannot be caused, otherwise it couldn't be "first". That means that the first cause must be unique in that it is uncaused, or eternal. The name we give that first cause is God.
We're still considering arguments for God from contingency. Next, we'll consider two more angles on this same general form: arguments from possibility and necessity, and from gradation of being.
In the mean time, you can hear more on the subject from Prof. John Lennox in his talk "Is Anything Worth Believing In?" Dr. William Lane Craig addresses a related bad objection in this snippet of "Arguments So Bad I Couldn't Have Made Them Up". Dr. Peter Kreeft also addresses objections to first cause in a brief video, "The Absurdity of Denying the First Premise of the Kalam Cosmological Argument". (We'll directly discuss the Kalam argument soon.)