Smoking is considered a cause of lung cancer because it significantly increases the risk of developing the disease. Most diseases, including lung cancer, are multifactorial, meaning they result from the interaction of various factors including genetic, environmental, and lifestyle factors. A cause in this context increases the probability of the disease occurring. It is not always a singular, direct cause but often one of many contributing factors. There are many "types of causes":

 - **[Necessary Cause][1]**: A factor that must be present for a disease to
   occur. Without it, the disease cannot develop. For lung cancer,
   smoking is not a necessary cause because lung cancer can occur in
   non-smokers. 
-  **[Sufficient Cause][2]**: A factor that alone can cause the
   disease. Smoking is not a sufficient cause because not all smokers
   develop lung cancer. 
-  **[Component Cause][3]**: A factor that contributes to
   the development of a disease. *Smoking is a component cause because it
   significantly increases the risk of lung cancer*.

Causation is often understood through counterfactuals: "If A had not occurred, B would not have occurred." In the context of smoking and lung cancer, if we imagine a world where no one smokes, the incidence of lung cancer would be significantly lower. This establishes smoking as a cause of lung cancer.


  [1]: https://en.wikipedia.org/wiki/Necessity_and_sufficiency#Necessity
  [2]: https://en.wikipedia.org/wiki/Necessity_and_sufficiency#Sufficiency
  [3]: https://en.wikipedia.org/wiki/Component_causes