The word "observe" originates from Latin, from the roots "ob" (over) + "servare" (to watch): thus its meaning is literally to watch over. The question is: what do we mean by watching over?
There is watching in the sense:
1) when you are in the flight and fight response (when the brain has stopped identifying)
2) when you are interpreting data from the eyes causing the identification of objects etc.
The observer is either whole or fragmented: as the whole, the observer is the observed; as the fragmented, the observer is memory which attention passes over.
For instance: if you wish to measure a straight line, there must exist the image of a line in memory, and the technical knowledge to measure it. If there is no image of a line in memory, no identification is possible, until the brain has focused upon the line and identified it as separate from the piece of paper. If there is no measurement, the line does not exist, but then, neither does the observer. Thus: when the observer is not operating (the brain), all that the mind feels is the flow of images, yet there is no identification.
We cannot understand what it means to be the observed, because when the brain is not the observer, nothing is recording. Thus in that trifle: it is possible for the observer to be absent, but not possible for the observer to know that it is absent.
In other words: the only way for the observer to be absent (one with the observed) is if the observer (brain) is not comparing/contrasting e.g. identifying, but if this happens, no memory of it happening will be recorded.
This strangeness has implications for quantum physics. When measuring very small particles, the measuring device effects the measurement: e.g the observer causes the effect, implying that the effect may not exist until it is measured.