The Four Grades of Vision:
The Blessed Buddha said:
In this some recluse or priest by means of alert & energetic effort
enthusiasm, proper rational attention & concentrated focus reaches
absorption on this sole thought: In this foul body of disgusting things
enclosed by skin, there are head & body hairs, nails, teeth, skin, flesh,
sinews, bones, marrow, kidneys, heart, liver, pleura, spleen, lungs,
mesentery, intestines, stomach, excrement, bile, lymph, pus, blood,
sweat, fat, tears, tallow, spittle, snot, urine & joint-fluid.
This is the first attainment of Vision...
Having done this and gone beyond it, he regards & perceives any body,
own & others, as only a set of bones covered with flesh, vessels & skin.
This is the second attainment of Vision...
Having done this and gone further, he comes to understand & directly
experience this continuous sequence of discrete conscious moments
established & manifesting both in this world and the other worlds.
This is the third attainment of Vision...
Finally, having done this & gone even further than beyond that, he comes to
understand & directly experience this unbroken stream of consciousness
neither established nor manifesting in this world nor in any other world.
This is the fourth attainment of Vision...
Source: The Exhaustive Speeches by the Buddha. Digha Nikaya III 105-5