I will attempt to make sense here, but don’t get your hopes up. I can feel as I write what follows that I am being neither eloquent nor clear, that my subject is defeating me. Nevertheless I will try.
It is necessary to assume the reality of cause and effect. Although, as Hume demonstrated, there can be no proof — nor even any evidence — that there is such a thing as causation, any philosophy that dispensed with the concept would be a dysfunctional philosophy, one incapable of doing what a philosophy is supposed to do, namely, to serve as a guide to action. One could not say that such a philosophy would be “false” necessarily, but it would be wrong and unacceptable at a level deeper than, and prior to, the level at which the concepts of truth and falsehood become relevant. As John R. Harris says in Climbing Backward Out of Caves, “causation is not a sight but a way of processing things seen” — but despite the lack of, the impossibility of, evidence for causation, it is nevertheless philosophically necessary for us to process things in this way.
The concept of mechanistic causation is incomplete without the concept of agency. Without agency, events are caused, but nothing is caused by anything. Each link in the chain of causation merely passively passes on what it has received. Such a model of the world — all dominoes and no fingers — is incoherent and philosophically useless. In a world with no agency, there can be no real causation, either. Such a world would contain intelligible patterns — it would not be a mere chaos — but it would be an abuse of language to say it contained causes and effects. And causes and effects are philosophically necessary.
What is agency, then? Agency causes things but is not caused by things. It has a relationship with prior events which is intelligible — it is not mere randomness — but does not amount to cause-and-effect. What exactly that means is admittedly hard to grasp. The concept of agency is (to borrow a simile from G. K. Chesterton) like the sun; it illuminates other things, but looking at it directly is not advisable.
Agency is what makes it possible for contingent things to exist. Without it, it would be impossible for necessary things to give rise to contingent things.
I should mention that, as a sort of corollary to my acceptance of agency, I also now accept the existence of God. I hasten to add, lest my religious friends get too excited, that at this point my tentative theism/deism is of the barebones god-of-the-philosophers variety — i.e., I don’t yet believe in anything that looks much like Jehovah — but it is nevertheless a significant step for me. This card-carrying atheist has turned in his card.