The ability to observe refers to the phenomenon of consciousness not a brain function (Block, 1995), so it doesn’t require any sense, thought or feeling. Damage to the visual cortex causes blindness but doesn’t stop consciousness, as people with locked in syndrome are still conscious. People born with no cortex are conscious (Merker, 2007) so it can’t depend on a cortical area. No brain area has been identified as the seat of consciousness, because it can persist even when the cerebellum, amygdala, hippocampi or cortex fail. The ability to observe is just there in a way that doesn’t require any particular brain function. It can apply to any sense, memory or feeling, so James concluded in 1892 that consciousness is a fundamental fact:
“The first and foremost concrete fact which everyone will affirm to belongs to his inner experience is the fact that consciousness of some sort goes on.” (James, 2019)
In scientific terms, this fact is valid because anyone can confirm that they observe and it is reliable because others can repeat the experience. That we each observe differently is irrelevant to the fact that we do observe. Without an observer there is no first person, so we would say “It is red” not “I see red”. To say I see or I hear implies an observer.
We know that we observe phenomena but we assume that it really exists (Kant, 2002). I know that I observe but I assume a physical world out there. I know with absolute certainty that I observe, but everything else is just an assumption, or as science says, a theory.
In physics, both relativity and quantum theory need an observer, one to provide the observer reference frame and the other to trigger a quantum collapse. Science is based on observation, so it is no surprise that the ability to observe is fundamental, as every fact depends on it. In our lives, and in science, the first fact is that we observe because without it, no other facts are possible.