Can a necessary but insufficient condition for something be a cause of that thing?

If it is an "insufficient condition" then it can be "a" cause, but not the cause.

Dr Dick

That makes sense.

I Googled my question and hit a web page on the work of Worthington (of the Ayn Rand School of philosophy I think). His logic would conclude that, under the circumstances posed by my question, opportunity is "causally related" but cannot be the - or even "a" cause. See: http://rationalargumentator.com/issue44/Worthington3.html

So I suppose it depends whether you accept the absolutist position of Worthington or not?

Worthington writes:

Entities do not act in a vacuum; they act under specific conditions. Under a given condition, they will act in a certain way—provided that the condition exhibits one of three causal relationships to the action. The law of causality does not specify the content of a given action or condition; rather, it “tells you that every action has a specific explanation” (65). The law of causality also stipulates the form which causal relationships may take.
The condition may be causally related to the action in one of three ways:
1) The condition is essential to the action, but not enough for it to occur (necessary, but not sufficient).
2) The condition is enough for the action to occur, but not essential to the action (sufficient, but not necessary).
3) The condition is both essential and enough for the action to occur (necessary and sufficient).
In cases 1) and 2), the condition is causally related to the action, but it is not a cause of the action, nor is the action an effect of the condition. When a condition is essential, but not enough, one needs to narrow one’s focus to find the condition which is both necessary and sufficient for the action to occur.

How's Law School going?

Either sufficient or insufficient...you chose.

We ought, I think, to distinguish between "a cause" and "the cause" of an event. A necessary condition is never "the cause" of an event. But it can be "a cause" of an event. For example, the presence of oxygen is a necessary condition of combustion, so it is a cause of combustion. But clearly the presence of oxygen is not the cause of combustion since its mere presence is not enough to cause combustion.