Tell that to your secretary as you DICTATE a letter to her!
"Tell that to your secretary as you DICTATE a letter to her!"
Which I then read to ensure that the letter indeed says what I intended it to say, and then I sign the letter, authenticating it as my writing...
...and there is no signature on the Gospel of Mark, therefore, it is not authenticated, and is therefore not eyewitness testimony of any sort, and is therefore Mark's hearsay (assuming Mark actually wrote the thing). Again, QED.