# Why a system is causal if the impulse response is zero for $t<0$?

I was reading a definition that

Suppose $$\h(t)\$$ is the impulse response of any system $$\Y\$$, then the system is causal if and only if $$\h(t)=0\$$ for $$\\forall t<0\$$

I know what a causal system is, but I don't understand it in terms of the impulse response. And I was wondering why is it causal here?

## 1 Answer

Causal systems are systems where the output only depend on past/present inputs. The impulse happens at time t=0, so if the output isn't zero prior to t=0 then it depends on a future input.

• Thank you very much – Mashpa Apr 17 at 20:09