Definition most often starts with a large group and proceeds to identity an individual at it's extreme. Carbon life form or not, Animal or plant, Human or other, Large or small, Female or male, dark or light. These are the primary groups that humans may well have in common.

Yet there is no end in the attempt to define an individual, an apple may well be Carbon, Plant, smallish etc, but an individual apple, although clearly unique is harder to identify from all other apples.

Words are also hard to define as their location in a string of words changes the meaning and even it's position was static the observer is not. Even a relationship between two is variable so identity, being a comparison is not set and will always be at best a degrading measurement.

The real problem isn't that change is inevitable, though it be so, but that the tools for observation are so limited that time is used to rough out the definition. For at any given moment there is no change and identity exists, but cannot be communicated. Any attempt to record the present must fail.