Logic allows philosophers to think seriously about abstract concepts in ways that are usually confined to the realms of science fiction or fantasy. Logical possibility is constrained only by the laws of logic, empirical possibility is constrained by the laws of nature, and technical possibility by the current state of human technological achievement. Logical possibility is different from that which is conceivable or imaginable. Probability can be mathematical, for example, where there is an equal chance that throwing a coin will ultimately lead to an equal distribution of heads and tails, or statistical, where forecasting is based on the relative frequency found in past data. The two processes of probability tend to mix. Mathematical probability requires a possibly infinite length of future time in which to work out, statistical probability never makes probable the specific case.