 In some theories, the time at which an event occurs or a property is instantiated plays an important role. In such cases, it will be useful to include a subscript indicating the relevant time. Thus the relata might be represented by \(C_t\) and \(E_{t'}\). If the relata are particular events, this subscript is just a reminder; it adds no further information. For example, if the event in question is the opening ceremony of the Rio Olympic games, the subscript ‘8/5/2016’ is not necessary to disambiguate it from other events. In the case of properties or event-types, however, such subscripts do add further information. The time index need not refer to a date or absolute time. It could refer to a stage in the development of a particular kind of system. For example, exposure to lead paint in children can cause learning disabilities. Here the time index would indicate that it is exposure in children, that is, in the early stages of human life, that causes the effect in question. The time indices may also indicate relative times. Exposure to the measles virus causes the appearance of a rash approximately two weeks later. We might indicate this time delay by assigning exposure a time index of \(t = 0\), and rash an index of \(t = 14\) (for 14 days).