Me, myself, and I

i) A stock objection which unitarians raise to Trinitarian theology is that in the Gospels, Jesus talks to the Father or prays to the Father. Unitarians point to that and exclaim: "Ah ha! Jesus and the Father must be different! How silly to think God is talking to himself!"

ii) Now, at one level, that's a brainless objection because Trinitarian theology requires the Father and the Son to be different in a fundamental respect. So that's hardly inconsistent with Trinitarian theology. To the contrary, that's what we'd expect if it's true.

iii) But here's another point: there's nothing necessarily incongruous about the same individual addressing himself or talking to himself. Suppose a time-traveler in the future writes a letter to himself in the past. In contains important information. Reading the letter will avert a disaster. Based on his advance knowledge of how the past will eventuate, he plants the letter at a time and place where his younger self is sure to find it.  

Whether or not time travel is incoherent, there is nothing incoherent in principle about sending a message to yourself. There's nothing contradictory about the idea of doing that. This is just a graphic way to illustrate that principle. 

iv) Or, to take a more prosaic example, suppose you keep a diary because you don't want to forget what happened to you, from day to day. Even though daily incidents might seem to be insignificant at the time, some might prove to be highly significant in retrospect. 

Every so often you review earlier entries in your diary. The diary was written by the writer to the writer. The diarist is both writer and reader. Although it's possible for someone else to read your diary, the reason you kept a diary was to remind yourself of things that happened to you. The diarist is the intended reader of his own diary. 

v) Or take a two-way correspondence between you and your counterpart in a parallel universe. Suppose the multiverse is real, and it's possible to communicate with yourself in the alternate timeline. So you and your counterpart in the parallel universe exchange messages, back and forth. 

Even if there is no multiverse, or even if, assuming there is a multiverse, communication between you and your counterpart is technically impossible, there's nothing contradictory about the idea of doing so. Once again, this is just a graphic way to illustrate that principle. 

vi) I'd add that (v) raises tricky issues about personal identity. Yet that's no different in principle from counterfactual scenarios generally. Many philosophers, unless they are diehard reductionists, make allowance for a certain amount of give in personal identity to accommodate the truth-value of counterfactuals. The price for denying that is extremely high.