Informally, an event such as a roulette wheel showing a particular number, happens at random if there is no way of telling in advance when it is going to occur. This might be construed metaphysically, meaning that there is no fact of the matter in existence at an earlier time that determines its occurrence. Or it might be construed simply as a limit on our knowledge, with no implications of determinism (see also chaos). The mathematical sign of randomness in an event would be that in an infinite sequence of trials, there is no effective method of selecting just those on which the event occurs, i.e. no recursive function determining the members of the sequence corresponding to the event. See also collective, exchangeability.