What scripting reference am I missing?

everything that I make has a load of compiler errors. it says I am missing an assembly reference 9"Assets/Standard Assets/Effects/ImageEffects/Scripts/Antialiasing.cs(20,33): error CS0246: The type or namespace name `PostEffectsBase’ could not be found. Are you missing a using directive or an assembly reference?") it seems as though every single script is “infected”. often I trawl through and delete the “infected” files, although it seems that the whole project will be deleted.

I imported the Legacy Image Effects package from the Asset Store, this package contained the PostEffectsBase classes

You are actually missing a scriptfile (maybe multiple) that contains the “PostEffectsBase” class. You probably imported only a part of an imageeffect without the required dependent files or you deleted them accidentally.