When the first stage of insight-knowledge arises panna realizes the
difference between nama and rupa through the mind-door. Cittas that
experience objects arise in a series or process of cittas. There are
processes of cittas that experience an object through one of the sense-
doors and processes of cittas that experience an object through the mind-
door. For example, visible object is experienced by cittas arising in the eye-
door process and after that visible object is experienced by cittas arising in
a mind-door process. When seeing arises, it seems to last, at least for a
while. It may seem that there is seeing without interruption. In reality visible
object, after it has been experienced through the eye-door, is experienced
through the mind-door. It seems that seeing is immediately followed by
hearing but in reality there must have been processes of citta in between.
However, we do not notice this, cittas arise and fall away very quickly.
Rupas that are sense objects such as visible object or sound, can be
experienced through their corresponding sense-door and through the mind-
door, and nama, such as seeing or hearing, can be experienced only
through the mind-door. Thus, through the mind-door both nama and rupa
can be experienced. When the difference between nama and rupa is
realized by panna, it is realized during a mind-door process. At those
moments it is also known what a mind-door process is. At this moment we
know it in theory, but at the moment of insight-knowledge it is directly
known. At such moments there is no self, no world, and nama and rupa
clearly appear one at a time through the mind-door. 
It is useful to know about the stages of insight-knowledge, otherwise we may
erroneously believe that stages of insight have arisen already. Panna is
bound to be weak in the beginning. There may sometimes be awareness of
a rupa and sometimes of a nama, but we should not take our weak
knowledge for the first stage of insight-knowledge. There is likely to be an
idea of self who is mindful. Is it already clearly known what the mind-door is?
It is beneficial to know what one does not know yet.

Topic 226