Causality
(1)
\begin{align} y[n]=\sum_{k=-\infty}^{\infty}h[k]x[n-k] \nonumber \end{align}

Namely $y[n]$ does not depend on $x[n+k] \hspace{0.2cm}\forall k>0$ and $x[n-k] \hspace{0.2cm}\forall k<0$.Clearly, $h[k]$ must be zero $\forall k<0$ as shown in figure.