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

Question

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?

Answer 1

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.