An intermediate cause is a node on the causal chain running from a problem's root causes to its symptoms. How this works is shown in the Problem Solving Map.
Why this is important
All problems have causes. Our search for a solution leads us to search backward along the causal chain from symptoms to causes. The closer we get to the root causes, the better the solution.
If your solutions are failing then it's usually because they are superficial solutions that are not resolving specific root causes. You have stopped at intermediate causes and are trying to solve those. If your solutions are succeeding it's because they are fundamental solutions that resolve root causes.
How can you tell if a solution to a difficult problem is superficial or fundamental?
Fundamental solutions change something deep within the structure of the system with the problem, like the way invention of democracy added The Voter Feedback loop or the way invention of electricity solved the transfer of universal power problem. Fundamental solutions are elegant at first glance. They are also usually a surprise, because root causes in difficult problems are well hidden. Fundamental solutions once applied shift the system into a new mode, one that is relatively easily sustained.
In contrast, superficial solutions tend to require lots of effort, require constant fiddling to make them work, only solve the problem partially or poorly, and tend to work only for awhile. All this is because superficial solutions try to do the impossible. They try in vain to solve intermediate causes.
The next time you see a solution in trouble, ask yourself: Is it possible this is a superficial solution? What causes is it trying to solve? Are those intermediate causes or root causes? Where's the analysis behind the solution? Does it contain a model of the problem that identifies the high leverage points the solution pushes on, and how those leverage points resolve the root cause? If not, the solution is guarenteed to be superficial and will fail.