The idea that the observer is not a "passive" entity but rather an active part of what shapes his reality is shown by Schopenhauer's argument (Crary, 74) in which he states that "what occurs within the brain... is wrongly apprehended as occurring outside the brain in the world" that is to say; it is not that there is a world which we observe, but rather what we observe becomes the world. This idea does not emerge solely on a philosophical idea of individualism but is also backed by the extensive studies that show that colours for example, are not seen the same by everyone and, as opposed to mearly existing are "physiologically" perceived (Crary, 74). This new view on the observer as an active entity created a new area of study, one with "perceptual autonomy" (Crary, 79).

A good example of the physiological nature of perception is is beauty; "The sight of a beautiful object... is also a  phenomenon of the brain"(Crary, 84). This discovery along with that outlined by Muller (that the perception of light is not necessarily dependent on the presence of light (Crary, 90) proves that the observer, through voluntary and involuntary processes determines what his environment is, based on what he, at the time perceives. The notion that the world is there to be absorbed by passive entities is now outdated and overly simplistic, being replaced with a comprehensive view on the observer. This observer, creator of his own world, is the modern observer. 

This image shows how the way we're used to seeing shadows and the spatial position of objects might change the way we see colours or shapes, despite them being "objectively" different from what we see.