I see
I understand or comprehend (something). I see, you have a doctor's appointment and need to leave a little early today—that's fine.
I see
Also, I see what you mean. I understand, as in I see, you'd rather go running in the morning while it's cool, or It's too early to run an ad? I see what you mean. This idiom uses see in the sense of "perceive" or "comprehend," a usage dating from 1300. Also see as far as I can see.