# Causal question on a discrete signal

Given a signal with a list of discrete values 1,2,3 (t=0),0,1,

Whether the signal is causal when the arrow is in 3 and why? I know the definition of causal is that the output will not depend on future input, but how to apply the definition in this question?

• A system is called causal if the system output $y[n]$ at time $n$ does not depend at all on the input values $x[n+1], x[n+2], x[n+3], \cdots$ all of which are still in the future at time $n$. A signal does not have an input or output and so it cannot be said to be causal. Commented Feb 15, 2015 at 22:14
• @DilipSarwate: I agree with you. Nevertheless it is quite common to speak of causal signals as defined here. Commented Feb 15, 2015 at 22:25
• Thanks. I understand now by check the link which @DilipSarwate mentioned above.
– Joe
Commented Feb 15, 2015 at 22:41
• @MattL.I have a following question. How did you use the theory which defines the system causal to explain the definition of causal function in the link? Thank you!
– Joe
Commented Feb 15, 2015 at 22:51