A causal system is a system with output and internal states that depends only on the current and previous input values. A system that has "some" dependence on input values from the future (in addition to possible past or current input values) is termed an acausal system, and a system that depends "solely" on future input values is an anticausal system. Acausal filters, for example, can only exist as postprocessing filters, because these filters can extract future values from a memory buffer or a file.