I know that when you are speaking of something a dead person wrote, you say “Mr. Deadperson says…”. I have an interesting situation, though, when a dead person is commenting on another dead persons writings, which are rather ambiguous. Do I say “Mr. Deadperson interprets Mr Otherdeadpersons’ writings as…” or do I use the past tense interpreted?
Hope that wasn’t too convoluted 
I would certainly hope that Mr. Deadperson is no longer actively commenting on Mr. OtherDeadPerson’s writings. They’d have to be pretty bad to elicit comments from beyond the grave. Stick with the past tense for things that happened in the past.
Ethilrist, i’m pretty sure you say that sticking to the past tense is not the rule when it has occured in someones writings. I’m only pretty sure about this when using the specific word “says” over “said.”
Also, I just happened upon a usage of “writes”, eg, “Mr. Deadperson writes,” so i’m just not sure if this tone of the present is used in all words.
I would use the simple past for Mr. Deadperson and the pluperfect (had as the helping verb) for Mr. Predecesor:
Plato wrote that Socrates had taught that all men are Greek.
St. Paul said that Jesus had said, “As often as you do this, remember me.”
They’re dead. They do not write or say or interpret things in the present. Although, one has to forgive preachers who do not view certain figures to be gone and without current influence, as in, “Jesus said ‘love your enemy’ and he teaches us (or, continues to teach us) how to do so by his example and through his Spirit.”
Also, if one is examining the text of a dead author, one may refer to what the text “says” in the present, since one is reading it in the present. “The constitution guarantees the right to bear arms. It says so very clearly. And I choose to ignore the rest of that sentence.”
Peace.
I would use the historical present. I was trying to think of an analogous situation I’ve encountered…in the intro to the second half of Don Quijote, Cervantes makes fun of an author (Avellaneda) who had written an apochryphal version…IMO, I would describe Cervantes’ actions in the present, and Avellaneda’s in the present or present perfect.
I would continue to avoid the past tenses…out of habit, and to avoid creating unweildy past perfects.
Here’s what Bartleby had to say.
I’m really confused now! 
I’m a senior English major; I’ve encountered this a lot.
Stick with the present tense as long as your talking about writing. For example, “In this essay, Descartes says that Plato, in his Republic, paints an unrealistic picture of human nature.” Or, “This essay mentions that The Republic defines justice very carefully.” You use the present tense twice because it’s talking about writing both times.
On the other hand, if you’re switching from talking about writing to talking about historical occurances, use the past. For example, “In this essay, Descartes mentions that Plato founded the Academy.” Or, “This essay says that Plato visited Socrates daily after Socrates’s arrest.” First, present tense for talking about writing. Second, past tense for talking about history.
Clear as mud?
Note: Descartes and Plato are names I just pulled from the air.