Another common type of reasoning is the search for causes and results.We want to know whether cigarettes really do cause lung cancer, what causes malnutrition, the decay of cities, or the decay of teeth.We are equally interested in effects: what is the effect of sulphur or lead in the atmosphere, of oil spills and raw sewage in rivers and the sea, of staying up late on the night before an examination? Causal reasoning may go from cause to effect or from effect to cause.Either way, we reason from what we know to what we want to find out.Sometimes we reason from an effect to a cause and then on to another effect.Thus, if we reason that because the lights have gone out, the refrigerator won't work, we first relate the effect (lights out) to the cause (power off) and then relate that cause to another effect (refrigerator not working).This kind of reasoning is called, for short, effect to effect.It is quite common to reason through an extensive chain of causal relations.When the lights go out we might reason in the following causal chain: lights out--power off--refrigerator not working--temperature will rise--milk will sour.In other words, we diagnose a succession of effects from the power failure, each becoming the cause of the next. Causes are classified as necessary, sufficient, or contributory.A necessary cause is one which must be present for the effect to occur, as combustion  is necessary to drive a gasoline engine.A sufficient cause is one which can produce an effect unaided, though there may be more than one sufficient cause; a dead battery is enough to keep a car from starting, but faulty spark plugs or an empty gas tank will have the same effect.A contributory cause is one which helps to produce an effect but cannot do so by itself, as running through a red light may help cause an accident, though other factors -- pedestrians or other cars at the crossroads -- must also be present. In establishing or refuting  a causal relation it is usually necessary to show the process by which the alleged   cause produces the effect.Such an explanation is called a causal process. Your refrigerator is not working and you have found that the electric power has been cut off. The power failure is a  _
necessary cause sufficient cause contributory cause none of them