# 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?