Explain why the system $\displaystyle y[n]=\sum_{k=n-5}^{n+1}x[k]$ is not causal?
Okay, I understand the concept of causality but not how to phrase the answer to this problem. I thought this was non-causal because the value of $k$ is independent of the input $n,$ but since it's only the current input and not a future input (it's not $n+1$), so I'm wondering what does make this non-causal?