The Dhyana paramita Sutra says:
‘For as long as you direct your search to the forms around you, you will not attain your goal even after aeon upon aeon; whereas, by contemplating your inner awareness, you can achieve Buddhahood in a single flash of thought.’
– The Zen Teaching of Instantaneous Awakening by Master Hui Hai, John Blofeld translation
Because Zen meditation is seeking the “Who?” that is aware of the forms, not the forms themselves. This is why in Chan and Zen, cultivators are asked to recite the Buddha’s name as a base for calming the mind – THEN, they are asked to investigate, “Who is it that is reciting the Buddha’s name?”
Seeking in forms is seeking appearances. Appearances are what is perceived, not the perceiver.
Appearances are part of the world of the not self because when you see appearances, it is you perceiving something other than you. Therefore whatever you see is not you.
So if you search for who you really are within the realm of forms or perceptions – it will be futile because it will always be in the realm of the perceived.
But the Buddho, the “One who knows” is the perceiver, not the perceived. It is the knowing. That is the real Buddha.