Verbal knowledge devoid of substance is fancy or imagination- this very truth gains a divine diction amidst the comprehensiveness of the ninth yoga sutra of Patanjali Yoga, which has been named as Sabdajnana anupati vastusunyah vikalpa.

sabdajnana verbal knowledge
anupati followed in sequence, pursued, phased in regular succession
vastusunyah devoid of things, devoid of substance or meaning
vikalpah imagination, fancy

Verbal knowledge devoid of substance is fancy or imagination.

Vikalpa, a vague and uncertain knowledge stands for playing with fanciful thoughts or words, and living in one`s own world of thoughts and impressions which have no substantial basis, and which does not correspond to reality. In such a state of hallucination, one is similar to the hare in the fable that imagined it had horns.

If vikalpa is brought down to the level of factual knowledge by examination, trial, error, and discrimination, it can arouse a craving for correct or true knowledge, and delusion can be transformed into vision and discovery. Unless and until such a transformation occurs, knowledge based on imagination remains without substance.