The point they're trying to make is that being able to imagine something without its defining characteristics does not make that thing possible in reality. For example, imagine red except with none of the wavelengths of red. Does that mean red is a separate concept from 625-740 nanometre wavelengths? No. It means you're imagining something that logically cannot exist.The point they're trying to make is that being able to imagine something without its defining characteristics does not make that thing possible in reality.

For example, imagine red except with none of the wavelengths of red. Does that mean red is a separate concept from 625-740 nanometre wavelengths?