Lines aren’t real. Everything doesn’t turn to ice the moment the thermostat hits 0.0000* C. We still understand that that delineation is very useful. Class is far less exact. Liberals think we propose that there is a very fine and exact line where everything to one side of it is saintly prole and the other side is evil bourgeois pig. Dialectics acknowledges that there is fluidity. There are many classes and subclasses with various moving parts and their own contradictions. Class relations are always changing and we know class traitors and those who do not understand their interests exist. But we understand that generally under certain circumstances we can define certain general categories the have certain interests according to conditions generally shared.

For another comparison, let’s say you said the exact same thing but about gender. “We know that men and women exist and will give you distinctions about how they work and act.” We know that this statement ignores huge complexity and conditionality. Are revisionist might hold that there is absolute bourgeois and proletariat and there are absolutely two genders. This understanding can be functional enough in many situations, but we both know that such ideas can be further analyzed with the dialectical method to reveal a much truer and more complex picture.

To organize a revolution we don’t say “hey workers, it’s in your interest to go kill your boss, so do it.” If it were so clear cut everyone would just be conscious of their interests and we’d have a stagnant communism already. But dialectics is how the world works, not simple slogans. We analyze all the relevant conditions through observation and existing concepts derived from practice to determine the best course of action within our material circumstance.

Edit: put another way, everything is too interconnected and changing to have a separate stable definition that corresponds to it, yet defining things relatively is very helpful