What is the difference between contemplative practice (vipaśyanā-caryā) and imagination?

Jia: Contemplative practice (vipaśyanā-caryā) is genuinely looking, genuinely observing, with all six faculties gathered on one thing and all participating. If the manas does not wander off, it can participate throughout, and the fruits of contemplative practice are recognized by the manas. Imagination is when the five faculties do not participate and only the mind consciousness conjures something out of thin air; the manas cannot participate and will not recognize it. 

Yi: The object of contemplative practice is based on objectively existing things. Imagination belongs to subjective conjecture. 

Bing: Contemplative practice is a mental activity centered on the manas; during the process, the mind consciousness provides the necessary guidance, allowing the manas to observe the object of contemplation with direct perception (pratyakṣa), just as it is. Imagination, on the other hand, is a mental activity centered on the mind consciousness, consisting mainly of the mind consciousness's erroneous cognition (non-valid cognition) of Image-Only mental sense-objects; it does not easily have a substantive influence on the manas. 

Ding: Imagination is the operation of the solitary mental consciousness, and the manas also participates; since the manas silently embraces all dharmas, it too has discerned them. Imagination belongs to the Image-Only Object and is non-direct, erroneous cognition. In contemplative practice, what both the mind consciousness and the manas take as object is a real-nature object, which belongs to the with-substance object, and is discernment by direct perception; it easily permeates and influences the manas, letting the manas realize the Dharma. Contemplative practice always makes both the mind consciousness and the manas engage in discernment by direct perception. Continuous discernment by direct perception (in which other faculties also participate) requires the power of meditative concentration (samādhi); only when the manas is concentrated can there be genuine realization. 

Wu: First, contemplative practice must be supported by a certain degree of meditative concentration; contemplative practice without the support of concentration falls into the category of imagination. Second, because contemplative practice is carried out in a state of concentration, the manas necessarily takes very few other dharmas as object and can focus its attention on the mental sense-object guided by the mind consciousness—that is, it becomes interested in the mental sense-object being contemplated. In imagination, the manas also participates, but it does not participate wholeheartedly; it will still be aware of other dharmas. Third, contemplative practice begins with guidance by the mind consciousness and gradually shifts to being led by the manas, which discerns the sense-object it attends to; the dharmas engaged are all states of direct perception, whereas the mind consciousness's imagination is erroneous cognition. Only states of direct perception better accord with objective reality and can bring the manas to firm conviction. In short, only when the manas genuinely attends and participates can its enormous capacity be brought into play, thoroughly transforming one's views and enabling correct choices. 

Which is easier—imagination or contemplative practice? Which is more effortless? 

Jia: Imagination is far easier; wandering thoughts come uninvited. It is known that many people deceive themselves and others, claiming they can practice contemplation and realize fruition without dhyāna, mistaking imagination for contemplative practice without realizing it. Once you understand the principles of cultivation, you will know that the vast majority of the time one relies on one's own effort to practice; there is no need to spend all day lingering in an ocean of theoretical books, absorbing others' opinions or views—counting another's treasures while in the end not receiving even half a coin for oneself.