The very first illusion is to believe that there is one, unique world of perception. The mind and the world we experience are inseparable, as it is the mind that makes the world meaningful. Our mind IS our world. Despite the fact that our mental construct of what is perceived is distinct from the objective reality, our mind accepts it as real.
Every organism, man as well as animal, lives in its own subjective spatiotemporal world that semiotic philosophers call ‘Umwelt’ (from the German Umwelt, ‘surrounding world’, or ‘life-world’). According to the biologist Jakob J. von Uexküll, organisms and their life-world shape each other in a functional loop (see fig. below): interactions between the subject and the outer world, mediated through the sense and effect organs, determine the world framework of the subject. Thus, a particular stimulus which has a perceptive cue or meaning to the subject induces always a purposeful reaction.
To explain this we can use the simplistic image of a sensitive robot. The most rudimentary example of a sensitive robot is the thermostat. This senses the temperature in a room; it determines the difference between the sensed temperature and the desired temperature and activates the heating element if the difference is too great. The capabilities of the ‘perception’ of a thermostat are the simplest imaginable. It can sense only one variable, temperature, and only two values for that variable: ‘temperature too low’ and ‘temperature high enough’. We can say, then, that ‘temperature’ is the life-world of the thermostat determined by the ‘too low’ and ‘high enough’ interactions. This is a very rough comparison because organisms have far more complicated perceptual systems. They are able to sense many variables independently and many different values for each variable, each triggering an appropriate action.
So we all live in the same world, but every category of organisms has its own life-world. Those self-contained, separate ‘realities’ coexist with our own and can be compared to parallel dimensions. Therefore, what may be perceptually relevant or important to one particular living being may pass unnoticed and is thus nonexistent for another one, and vice-versa. The same landscape looks – and to all intents and purposes is – quite different to a human, a cat and a bird respectively.
You can download here the full French version of Uexküll’s book “Mondes animaux et monde humain” (in German Streifzüge durch die Umwelten von Tieren und Menschen)