In his remarkable book The User Illusion, Thor Northrendars states: 90% of what we see, hear, feel - is generated within us and has nothing to do with objective reality. For a fraction of a second before consciousness begins to evaluate reality, the brain has already formulated its own attitude to this. It turns out that, having switched on, our consciousness evaluates not the objective reality, but the formed image provided by the brain.
You, in all probability, have heard about the so-called "blind spots" of our vision. In each eye there are such points. In our eyes there are photoreceptors and retina, which in symbiosis and provide us with the ability to see. It functions as a sensitive film - information that is transmitted through the rays of light from the outside world is recorded. Nevertheless, there is a point of attachment of the optic nerve to the retina - this is the very place and forms a gap in the whole canvas of visual perception. So, the brain gets zero information from this point. Therefore, we should see the picture surrounding us with "holes" ?!
But this for some reason does not happen. So our brain manages to insert the missing elements of the puzzle?
Back in 1781, Kant insisted that for man there are two worlds: "noumenal" (or objective, real) and "phenomenal," which is our version of the real world, but missed through personal perception.
We evaluate and interpret what is happening, based on our personal experience, our knowledge, habitual phenomena. There is a clearly expressed tendency to feel what we hope to experience. If we are sure that some actions can cause ridicule of colleagues, and not admiration, then we will everywhere see the scornful smirks of employees, even if in fact this is not in sight.
Therefore, we recognize that hallucinations are a phenomenon that we regularly encounter. This can even be called a special skill, developed by the brain in the process of growing up to assess the surrounding reality and facilitate the existence in this world.