Why is it easier to visualize something actually seen with one's own eyes than something imagined out of thin air?

What is seen with one's own eyes includes direct perception by the five sense consciousnesses and the mental consciousness accompanying the five sense consciousnesses (pañcavijñāna-sahagata-manovijñāna, C. 五俱意識), as well as direct perception by the manas. Because the mind consciousness has seen it before, it can be easily recalled by the mind consciousness. 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 cognitive recognition, and it relies entirely on the erroneous cognition/non-valid cognition of the mind consciousness's 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 meditative concentration to visualize it. 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 thinking of it day and night, cultivating the white bone contemplation becomes somewhat easier.