Why is it easier to visualize what one has seen with one's own eyes than to visualize from pure imagination?

What is seen with one's own eyes includes direct perception (pratyakṣa) seeing by the five consciousnesses and the consciousness accompanied by the five senses (pañca-sahagata-mano-vijñāna), as well as direct perception seeing by the manas. Because the mind consciousness has seen it before, it can more easily recall it. What is not seen with one's own eyes has not been experienced by either the mind consciousness or the manas; there is no direct perception cognition, and it relies entirely on the mind consciousness's non-valid cognition (non-valid cognition) imagination, so the manas cannot exert any force either—this is why contemplative visualization is rather difficult. In the past, those who cultivated the contemplation of the white skeleton had no actual skeleton to refer to and relied entirely on the power of meditative concentration (samādhi) for visualization. Nowadays there are many skeleton models available for direct observation as reference; as long as one finds a way to buy a model and observes it every day, practicing until one thinks of it day and night, cultivating the contemplation of the white skeleton becomes somewhat easier.