[R] Noob question - Identity argument within aggregate function?

[knavero]

>aggregate(z, identity, mean)
  1   2   3   4   5 
1.0 3.0 5.0 6.0 7.5 
> aggregate(z, mean)
Error: length(time(x)) == length(by[[1]]) is not TRUE

Can someone help me understand the error above and why "identity" is
necessary to satisfy the error




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