6 ideas
A blind, deaf tick climbs a twig and waits, sometimes for years, responding to only three cues. The smell of butyric acid from mammal skin makes it drop. The warmth of the body it lands on sets it searching, and touch of a hairless patch makes it bore in and drink. Everything else in the forest, all its richness, simply does not exist for the tick. Its life shows how a whole functional world can be built from a handful of signs.
An Umwelt is the subjective world an animal inhabits. It is made only of the features its receptors can register and its effectors can act on, not of the objective environment as a whole. Two animals in the same meadow live in different worlds, because each world is assembled from that species' own perceptual marks.
Every animal is joined to its world by a closed loop. A perceptual organ picks up a perceptual cue from an object, and the effector organ then imprints an effect mark on that object. The effect mark cancels or changes the perceptual cue, which ends one action and triggers the next. Behavior is therefore a chain of these circles, not a response to the full object.
A single physical thing becomes a different object depending on whose world it enters. The same oak is a shelter for a fox, a hunting ground for an owl, a road for an ant, and lumber for a forester. Meaning is not a property of the thing but of the functional tone an organism's needs assign to it.
Each animal's world has its own smallest unit of time, the shortest interval it can tell apart. Humans experience about 18 moments per second, which is why faster flicker blurs into continuous motion for us. A fighting fish perceives faster than that, and a snail slower, so a stick tapped 4 times a second looks still to the snail. Time, like space, is built by the subject rather than given as a shared, objective frame.
Uexküll argues that treating animals as mechanisms made of reflexes misses the central fact. Each animal is a subject that actively sorts signs into meaningful perceptual and effect marks. Biology must therefore study how an animal's world is constructed from the inside, not only how its body reacts to physical stimuli.