A causal system is a system that depends only on the current and previous inputs. This property is referred to as causality. A system that gets values from the future is termed an anticausal system (also called an acausal system).
Classically, nature has been considered to be a causal system. Though causality is still thought to be found in nature, discoveries in modern physics have challenged the view that nature is strictly causal. For an in-depth discussion, see causality (physics).