Smoking is considered a cause of lung cancer because [it significantly increases the risk of developing the disease][1]. 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][2]**: 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][3]**: A factor that alone can cause the
   disease. Smoking is not a sufficient cause because not all smokers
   develop lung cancer. 
-  **[Component Cause][4]**: 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/Lung_cancer#Causes
  [2]: https://en.wikipedia.org/wiki/Necessity_and_sufficiency#Necessity
  [3]: https://en.wikipedia.org/wiki/Necessity_and_sufficiency#Sufficiency
  [4]: https://en.wikipedia.org/wiki/Component_causes