A cause is something that brings about an effect. Clouds, for instance, can cause rain because clouds can bring about the effect of rain – they are the reason why the rain occurs. Exactly what it means for something to ‘bring out’ something else is subject to considerable philosophical dispute, but for our purposes the common-sense understanding of one thing making another happen is sufficient.
Causes can be either deterministic, meaning that they will always and invariably bring about their effects, or probabilistic, meaning that they only make their effects more likely to occur. For example, the force of gravity is a deterministic cause, resulting in two massive bodies experiencing a force of attraction to each other proportionate to the product of their masses. The force of gravity always causes objects to be attracted to each other in this way, without exception. The tendency of smoking to bring about lung cancer, however, is an example of a probabilistic cause, because smoking does not always produce lung cancer; it only increases the probability that lung cancer will occur.
Another distinction that can be made is between proximate and ultimate causation. A proximate cause is the direct cause that brought about some effect, whereas the ultimate cause is the underlying, more fundamental reason why the effect occurred. For example, the proximate cause of death may be a heart attack, but the ultimate or underlying cause might have been poor diet and lack of exercise, for this in turn is what brought about the heart attack. In many cases it is the ultimate cause which is of more interest and relevance than the proximate cause, and therefore care must be taken to continue investigating a phenomenon even after a proximate cause has been identified, to probe for possible deeper ultimate causes.
A single effect often has multiple causes. This typically is the case for complex events, such as diseases or historical occurrences, where many contributing causal factors all combine to bring about an effect that none of them singly would have been sufficient to cause. A single cause can also have multiple effects, as for example when the act of throwing a glass of water onto the ground causes the water to spill, and also the glass to break.
Causal reasoning: Philosophy Pages article on types of causal reasoning
Modern theories of causation: Discussion of some of the philosophical issues raised in understanding causation