Perceiving or not perceiving? My question is related to the quote below taken from Chapter 29 of The Diamond Sutra. Can you provide any references for other explanations for this quote or can you provide your own explanation?


  “Subhuti, if any person were to say that the Buddha is now coming or
  going, or sitting up or lying down, they would not have understood the
  principle I have been teaching. Why? Because while the expression
  ‘Buddha’ means ‘he who has thus come, thus gone,’ the true Buddha is
  never coming from anywhere or going anywhere. The name ‘Buddha’ is
  merely an expression, a figure of speech.”


As I understand from my current practise, there have been many instances I've been walking and I am aware that there is nowhere I'm walking to, and nowhere I'm walking from and no reference for somebody walking. I can then only discern beyond forms and see that there is only what I perceive to be a stillness in which there is neither time, distance and dimension. I don't know what is doing the perceiving in this instance or if anything at all is being perceived. If I am able to put this into a language there must be a perceiver beyond all forms or that my conceptual mind is doing this perceiving. In the latter case I would be being fooled by mental perceptions.

Out of curiosity and using consciously directed thought I experimented with this experience by watching a seagull flying through the air. Within several seconds I removed the earth and the rest of the universe. The seagull was bouncing its wings and orienting on the spot. From this I deduced that form provides dimension, distance and time. Without form the seagull cannot move from one position to another and time seems intrinsically bound to distance and dimension. That's as far as I got.

It may seem that I'm bordering on nihilism here, but on the contrary: there is the use of form to make the deeper discovery. Form seems to be the substrate to depth of knowledge but in this case it seems like I haven't learnt anything.
Another more precise and concise translation of the Diamond Sutra renders it differently, Chapter 29: 


  “Subhuti, if someone said the thus-come comes, goes, sits, or reclines, this person doesn’t understand the meaning of what I taught. Why is that? What thus-come is, that is from nowhere comes nor anywhere goes, hence called the thus-come.


I think you have quite good intuition to come to conclude that ...From this I deduced that form provides dimension, distance and time. Without form the seagull cannot move from one position to another and... 

This chapter, on the meaning of Tathagata, is touching on the Tathagatagarbha. Tathagata, generally renders as thus-come. What is Tathagatagarbha? It is formless, but from it form arises; it is emptiness, but it contains everything - the seeds. Hence is called the Tathagatagarbha (如來藏), the womb of the treasure of suchness.

When you perceive form, by it defines space/time, that is activity of the mind, which is a "negation" from the suchness. The form doesn't exist independently from your mind. In fact it is your mind limited by the 5 senses, the intellect (6th consciousness), the subjectivity (7th consciousness/ mānas-vijñāna), hence it perceives the world as it created - the forms. But it is hindered by it's own creation - the activities it engaged. When you can remove from all these activities, you are abiding in the suchness. The Buddha is abiding in the suchness therefore called the Tathagata - he doesn't engage in any activity. There are many names referring to the same ultimate reality in different contexts for different significance: Alaya, Dharmakaya, Citta (mind), the 8th consciousness... etc.  

It's too board to completely elaborate, especially a lot of Sutras to help understanding it are not available in English. But the above is the condensed favour hopefully you could enjoy :)

Just as a note, your linked English version of the Diamond Sutra is awfully translated, from the perspective of someone who is able to read the Chinese Classical Diamond Sutra. That appears the same as Red Pine's version but the site belonged to one called Alex Johnson. I wonder if the Alex Johnson has infringed someone's copyright. One the other hand, I hate to say this since Red Pine appeared genuinely liking oriental cultures, I read somewhere someone said Red Pine's translation can't be taken serious. He mixed the Tibetan text with the Chinese compiled as one. Not sure which 2nd language he is fluent in, Tibetan or Chinese? 

Translation of Sutra is a very very serious matter, if it is translated wrongly, it causes severe defilement to oneself, for it misleads the Dharma seekers - this is the attitude all ancient Dharma masters taken.