 Second, causal theories of mental content distinguish what has come to be known as natural meaning and non-natural  meaning.[3]  Cases where an object or event X has natural meaning are those in which, given certain background conditions, the existence or occurrence of X “entails” the existence or occurrence of some state of affairs. If smoke in the unspoiled forest naturally means fire then, given the presence of smoke, there was fire. Under the relevant background conditions, the effect indicates or naturally means the cause. An important feature of natural meaning is that it does not generate falsity. If smoke naturally means fire, then there must really be a fire. By contrast, many non-naturally meaningful things can be false. Sentences, for example, can be meaningful and false. The utterance “Colleen currently has measles” means that Colleen currently has measles but does not entail that Colleen currently has measles in the way that Colleen’s spots do entail that she has measles. Like sentences, thoughts are also meaningful, but often false. Thus, it is generally supposed that mental content must be a form of non-natural underived  meaning.[4]