Here is a passage from Ñanavira's Notes on Dhamma.
Images here refer to mental content (imaginations). Five-base refers to the five senses (sight, sound, smell, taste, touch)
There is no doubt that images are frequently made up of elements of past real [five-base] experience; and in simple cases, where the images are coherent and familiar, we speak of memories. But there are also images that are telepathic, clairvoyant, retrocognitive, and precognitive; and these do not conform to such a convenient scheme. The presence of an image, of an absent reality, is in no way dependent upon its ever previously [or even subsequently] being present as a present reality [though considerations of probability cannot be ignored].
So why is 'images are frequently made up of elements of past real [five-base] experience' a convenient scheme since it does not seem to fit with the dictionary definition of scheme? How is it a plan and what is it a plan for? Also why is it convenient, convenient for achieving what exactly?
Why does he say that 'considerations of probability cannot be ignored'?