Inner speech—what we say to ourselves when alone—provides a reasonably good outline of our conscious psychology, an outline of how we understand ourselves.
Inner speech may also include semi-conscious information and subject matter.
When engaged in personal art and poetry inner speech frequently draws on unconscious material, though it is difficult to know where to draw lines between that and psychology. Art is all but defined by its capacity to evoke many interpretations.
Inner speech wanders and can become subversive, even if beautiful, by confirming misunderstanding.
When we are consciously mindful of our inner speech and deliberately pay extra attention to it (a valuable practice), our speaking will change because whenever we strive or focus on anything our relationship to it changes.
When speech pays attention to itself, it brings recursion, one of its core features, to bear on itself. In doing that it raises self-awareness to higher macro-levels or causes self-awareness to view itself from different perspectives.
Consciousness seems to require consciousness of something; in this case it is consciousness of consciousness, a very simple thing actually.
Does water know it’s wet? I don’t know. Does consciousness know it is conscious? Of course it does. We must admit here, though, that what we are conscious of is often wrong.
Being wrong is a big problem with inner speech. I might be talking rather passionately to myself about something that never happened the way I have come to see it. We all know this, though it’s hard to know what to do about it.
That’s probably a big piece of what the Buddha meant by delusion, or even wrong speech. Mumbling away in my own head about something I am completely wrong about!
With an honest partner at least I can get an honest answer about whatever they are thinking right now and compare that to whatever I thought I saw in them. And from that I can tell whether what I thought I saw in them was right or wrong.
That is very good information, some of the best. Let a few seconds pass and their memory will already be eroding, their information not-so-slowly consumed by inner speech.
Friends will typically provide all the inner speech you want, but we would be back at square one if we took that in place much better information from as close as possible to the actual moment that just occurred.
If you think about it, you will probably agree that we can really only gain an objective insight into our psychology in the moment with an honest partner. And in those instances, we will mostly only gain insights into small bits of it.
Fortunately, with time and an accumulation of many small bits of information like that, we will see much better outlines of our psychologies than either our own inner speech can provide us or that can be provided by any theory that comes from outside.