What is called "voidness" is not an object, but a functioning. It is void because seeing cannot see seeing.
Appearing (form) is not an object, BUT A FUNCTIONING. It appears as an objectivisation.
To "empty," therefore, is to apperceive non-objectively (void-seeing): no thing seen as such by no see-er as such.
Doctrines also are ways of seeing, not things seen; they are not a description of absolute truths---for nothing of the kind could exist.
--- Wei Wu Wei on the Heart Sutra