Okay, tangent time...
My understanding of the collective unconscious (which is a bit hazy) is that it's a sort of inherited knowledge, something like psychological instincts (as opposed to physiological instincts). I believe Jung uses this theory to defend the validity of the introverted viewpoint. Extroverted functions have a connection to reality, so that they are validated by objects verifiable by others. The introvert does not have this, but they do have a closer access to the collective unconscious which gives some weight to their perspective/judgment; it's not just arbitrary memory or values or logic or future vision, but there's a universal aspect to the concepts behind them, concepts which are recognized by others as true, even if not easily verified by anything tangible. I think this is why people may say introverts are "deep" where extroverts are "broad".
To quote a humorous part of Psychological Types (which I featured in my blog recently, because it made me laugh for a good minute):
The extrovert may have trouble seeing the validity of introvert's thoughts. You see this all the time, where the introverted functions are criticized for their "subjectivity" as if subjectivity was somehow inferior, and not an important part of understanding the world. I notice this is often done from those with the same function in the opposite attitude; ie. Te & Ti or Ne & Ni. I think the introvert can devalue the extrovert view as "shallow" sometimes too; but more often, I think they are complementary .Originally Posted by Jung
Okay, now how this relates to Si is there is supposedly an instinctive awareness of what has been in the past, beyond the individual's own experience. This awareness makes the Si-dom gravitate towards images of the past. Hence, the association of Si with tradition and history. Their Je function will judge the significance of these perceptions, and will seek to protect that which may violate an awareness of what people have needed or what has worked down through time.
Ni, devalues these images of the past, preferring to see what they imply in regards to the future, as opposed to their direct meaning; in that sense, symbols are formed from these impressions. Ni has a synthesized vision of what will be, as opposed to what has been. So Si prefers the literal where Ni prefers the symbolic.
In a similar way, Se sees the object for what it is, and Ne sees what it implies indirectly, or could be, or the concept it connects to. So Se sees the outer object, and Si sees the inner object. Ne sees what the outer object implies, and Ni sees what the inner object implies, indirectly. From what I gather, the source of the inner object is partly the collective unconscious, but just as extroverts may all interpret the external object differently, so this will be interpreted differently by the introvert. Of course, in reality, no one thinks with just one function, so introverts also have their dom function influenced by their extroverted aux function.