Not-Buddhism is the highest level of Buddhism? I'm new here. I have a question regarding "Not-Buddhism" as the highest level of Buddhism.

I'm not sure where is the source, but this is the koan.


  Tozan said to his monks, "You monks should know there is an even higher understanding in Buddhism." A monk stepped forward and asked, "What is the higher Buddhism?" Tozan answered, "It is not Buddha."


What does Tozan mean by "It is not Buddha"?
The Aggi-Vacchagotta Sutta shows how the Buddha treats views about the cosmos being eternal and views about the fate of the Buddha after death.


  
    "Vaccha, the position that 'the cosmos is eternal' is a thicket of views, a wilderness of views, a contortion of views, a writhing of views, a fetter of views. It is accompanied by suffering, distress, despair, &amp; fever, and it does not lead to disenchantment, dispassion, cessation; to calm, direct knowledge, full Awakening, Unbinding.
  


And another quote,


  
    "A 'position,' Vaccha, is something that a Tathagata has done away with. What a Tathagata sees is this: 'Such is form, such its origination, such its disappearance; such is feeling, such its origination, such its disappearance; such is perception...such are fabrications...such is consciousness, such its origination, such its disappearance.' Because of this, I say, a Tathagata — with the ending, fading away, cessation, renunciation, &amp; relinquishment of all construings, all excogitations, all I-making &amp; mine-making &amp; obsessions with conceit — is, through lack of clinging/sustenance, released."
  


Having a position about what is Buddhism has similar drawbacks to having a view about the eternal-ness of the cosmos. Tozan is discouraging views like "This practice Buddhist" or "That idea is part of Buddhism".