You're getting this idea of observation mixed up with the senses. Yes, we can observe the external world (nature), but introspection (observation of the inner self) is something different entirely. Observation of nature deals with the physical while observation of the internal deals with the abstract like state of mind and virtue, which are the contents of the soul. My thoughts and emotions are unique to only me and are most surely not tangible; they are not external.

I agree that we cannot find this "ultimate truth", but you must also consider that all actions you have taken which you have deemed "correct," come from within. We all believe there is some truth to what we say and believe, else we would not say or believe it. Truth does exist, and it may exist only in our minds. Nothing in nature is explicitly telling us what is true and what isn't; we are the ones deciding based off of belief. You can absolutely believe that there is nothing we ought to do, and you can absolutely say that the goals of living are separate from the goals of observing, but I challenge anybody to live without a single thought about their condition. Surely, it is hard to deny that you do not have flaws and things which could be improved upon. I simply believe that basing your life upon attaining the unknowable is pure folly (as alluring as it is); there are much better things to concern yourself with. The same goes for proclaiming to understand things you clearly do not. You are only going to contort your perception of the world by thinking you know what you do not. We may be human, but that doesn't give us the excuse to act irrationally.

I think you make good points, but I think you might be distracted with the pedantics of the world (as we all are, though surely to differing extents). The most important thing in life is the betterment of the self and others, I believe. Truth is supportive of this goal, but not all-important. We can attain enlightenment through self-truth, but we do not need ultimate truth to realize this.