We might say you can boil this entire spiritual seeking thing down to two things - what exists and what appears. In fact a direction for a seeker might be to inquire into what actually exists and what is it that appears?
Maybe we get the sneaking suspicion that, due to inattention, due to the fact that we had these ideas indoctrinated before we had a chance to intelligently question them - we take the world to be a jumble of separate things, a collection of independently-existing things - the idea of a "thing" or "thingness" is critically important here. What is a "thing" anyway?
A "thing" is supposed to be something which exists, yes? So what does that really mean? Did that thing pop into existence? Where was it before? Where does it go when it ceases to exist? How did it come into existence anyway? We really don't want to think much deeper than this because it's maybe too complicated or we don't think it's the right direction. The challenge is to do it - inquire into this idea - give it a chance for doubt.
We have to be able to see the reality of dependent existence - in other words things don't just exist on their own - they are dependent on something - mostly they are just patterns - collections of something else - like molecules - so a tree is organic matter - molecules - elements - atoms - what we see as "tree" is just a pattern of energy - it is a "tree" to "eyes" - in other words "eyes" are needed to know "tree" - in this case "tree" is just a particular identified and familiar shape or pattern - if the means of knowledge were not eyes but ears, we might need the tree to make some identifiable noise, yes? So we can identify "tree".
In this case "eyes" are part of the equation "tree" - without "eyes" there is no "tree" - if the means of measurement was a microscope, it would not be "tree" but "molecules" that we would identify - yes? So the means of knowledge by which you are aware of that "thing" is part of the knowledge of that "thing", yes?
So what about the knowledge of your Self? How do you come about knowing your Self? What means of knowledge is required? Do you know yourself via the eyes? Do you see yourself? Do you hear yourself? Do you feel yourself?
In fact that which you know as "I" is the subject of all these objective situations - you cannot feel or see or touch the "I" - "I" is not available to you for objective measurement. It is that which enjoys the measurement yet itself is never subject to measurement.
So we can trace back any "thing" that we can know objectively - what is it made of? What is it's cause? What is it's source? Then again when we find a source we must again ask what is the source of that? What is the cause of that? Finally we may reach the end of our capability of identifying that ultimate source - we might come to the conclusion that the source of one is the source of all.
See if you can trace back your source in this way. Are you the result of some pattern? Are you the effect of some cause? Did you begin existing at one point where you did not exist before? Are you just another "thing-among-things"?
These are questions we must ask and really are the foundation of self-knowledge. If we take what appears equally to that which exists, we are overlooking the answer everytime. We might realize that what appears must come and go due to some essence or source which IS, which exists, and the form which comes and goes is never something new, never something other-than that source.
This is the revelation of Vedanta - Tat Tvam Asi - You are THAT - THAT is what-IS - what exists - there is only ONE.
Or better said - there is not-two.