These three requirements of causality can be said to summarize causal inference. Spurious variables are things you haven't thought of that might explain what you've found. In the end, there are usually more correlates than causes, and one cannot control everything. Causality is always an inference . This particular type of relationship must be inferred from the observed information, and then related back to known information.
Inference demonstrates itself in hypothesizing, sampling, designing, and interpreting.
It is the basis for scientific generalization, especially those having to do with the explanation of causality. It is never the final proof , but because final proof is itself never possible, inference is the best substitute. It enables ways to advance science, debunk mistaken beliefs, and is always mindful of its own limitations. Certain safeguards are built into the process which protect against unwarranted generalizations. The process of generalizing in an explicit and scientific manner is inference.