Visible object that was experienced by citta of a sense-door process has fallen away; sense-door processes and mind-door processes of cittas alternate very rapidly. Visible object impinges again and again and seeing arises again and again. When their characteristics appear we cannot count the different units of rupas of cittas that see, they arise and fall away, the impression (nimitta) of what is seen and of the seeing appears. If there are the right conditions a citta with sati (mindfulness) can arise that is mindful of its characteristics. However, mindfulness of seeing arises after seeing has fallen away, not at the same time as seeing.