SuttaCentral
The advice that, “In the seen there is only what is seen, in the heard, only what is heard… in the cognized, only what is cognized,” does that mean that when hearing, do not see, and when cognizing, do not feel touch, or does it mean something else?
Ud 1.10
For example, if in the seen there is only what is seen, are there other things besides the six senses that are not to be undertaken? Or does it mean like don’t be mindful of more than once sense at a time?
The next part of that lesson instructs Bahiya not to identify with or compound those things. If you see a rock, don’t describe the rock - just let the rock be seen and known directly. Some people see a rock and then start mentally describing and manipulating it, even identifying with it through thoughts such as “I like this rock” or “I want this rock” etc. If you have a negative thought arise there’s no reason to add to it by thinking things like “I should be ashamed of myself, how could I think that”, etc. The same with feelings. With all the senses, when contact arises we can train not to make a thing of what has arisen, including pain and pleasure, and in this training we can begin to disidentify with the aggregates for the goal of dispassion.
“When you have trained in this way, you won’t be ‘by that’. When you’re not ‘by that’, you won’t be ‘in that’. When you’re not ‘in that’, you won’t be in this life or the next or between the two. Just this is the end of suffering.”
… a passage from Sartre: ‘The eye (etc.) is that in the world by which one is a perceiver and conceiver of the world’.
Let us now try, with the help of Heidegger’s indications,[3] to tie up these two Sutta passages.
(i) To begin with, ‘I—here’ is I as identical with my senses; ‘here’, therefore refers to my sense organs (eye, ear, nose, tongue, body, and also mind). The counterpart of ‘here’ is ‘yonder’, which refers to the various things in the world as sense-objects. ‘Between the two’ will then refer (though Heidegger makes no mention of this) to consciousness, contact, feeling, and so on, as being dependent upon sense organ and sense object—cakkhuñca paticca rūpe ca uppajjati cakkhuviññānam, tinnam sangati phasso, phassapaccayā vedanā, etc. (Salāyatana Samy. 107: iv,87).[4]
(ii) In the second place Heidegger says that ‘here’ and ‘yonder’ are possible only in a ‘there’; in other words, that sense-organs and sense-objects, which are ‘amidst-the-world’, in Sartre’s phrase, are possible only if there is a world for them to be ‘amidst’. ‘There’, then, refers to the world. So the ‘here’ and ‘yonder’ of the Bāhiya Sutta correspond in the other Sutta to the ‘eye (and so on)’ as ‘that in the world…’.
(iii) But Heidegger goes on to say that there is a ‘there’ only if there is an entity that has made a disclosure of spatiality as the being of the ‘there’; and that being-there’s existential spatiality is grounded in being-in-the-world. This simply means that, in the very act of being, I disclose a spatial world: my being is always in the form of a spatial being-there. (In spite of the Hindus and Hegel, there is no such thing as ‘pure being’. All being is limited and particularized—if I am at all, I am in a spatial world.) In brief, there is only a ‘there’, a spatial world (for senses and objects to be ‘amidst’), if I am there. Only so long as I am there shall I be ‘in the form of being-amidst-the-world’—i.e. as sense-organs (‘here’) surrounded by sense-objects (‘yonder’).
(iv) But on what does this ‘I am there’ depend? ‘I am there’ means ‘I am in the world’; and I am ‘in the world’ in the form of senses (as eye…mind). And Heidegger tells us that the ‘here’ (i.e. the senses) is always understood in relation to a ‘yonder’ ready-to-hand, i.e. something that is for some purpose (of mine). I, as my senses, ‘am towards’ this ‘yonder’; I am ‘a being that is de-severant, directional, and concernful’. I won’t trouble you with details here, but what Heidegger means by this is more or less what the Venerable Ānanda Thera means when he said that ‘The eye (and so on) is that…by which one is a perceiver and a conceiver of the world’. In other words, not only am I in the world, but I am also, as my senses, that by which there is a world in which I am. ‘I am there’ because ‘I am that by which there is an I-am-there’; and consequently, when ‘I shall not be that by which’, then ‘I shall not be there’. And when ‘I shall not be there’, then ‘I shall neither be here nor yonder nor between the two’.
(v) And when shall we ‘not be that by which’? This, Heidegger is not able to tell us. But the Buddha tells us: it is when, for us, in the seen there shall be just the seen, and so with the heard, the sensed, and the cognized. And when in the seen is there just the seen? When the seen is no longer seen as ‘mine’ (etam mama) or as ‘I’ (eso’ham asmi) or as ‘my self’ (eso me attā): in brief, when there is no longer, in connexion with the senses, the conceit ‘I am’, by which ‘I am a conceiver of the world’.
Usually seeing the rock is immediately associated with recognising it as a rock. Of course if one is able to suspend thinking and naming things, that’s good 
.
But in the context of the Sutta, the main problem is existence of the subject. Bhava or being is supported by the appropriation of things, seeing them as “mine”. Rock can be recognised as rock even by arahat, so the main thing which should be avoided is to see rock as “mine”.
the puthujjana thinks ‘things are mine (i.e. are my concern) because I am, because I exist’. He takes the subject (‘I’) for granted; and if things are appropriated, that is because he, the subject, exists. The ditthisampanna (or sotāpanna) sees, however, that this is the wrong way round. He sees that the notion ‘I am’ arises because things (so long as there is any trace of avijjā) present themselves as ‘mine’. This significance (or intention, or determination), ‘mine’ or ‘for me’—see A NOTE ON PATICCASAMUPPĀDA [e]—, is, in a sense, a void, a negative aspect of the present thing (or existing phenomenon), since it simply points to a subject; and the puthujjana, not seeing impermanence (or more specifically, not seeing the impermanence of this ubiquitous determination), deceives himself into supposing that there actually exists a subject—‘self’—independent of the object (which latter, as the ditthisampanna well understands, is merely the positive aspect of the phenomenon—that which is ‘for me’). In this way it may be seen that the puthujjana’s experience, pañc’upādānakkhandhā, has a negative aspect (the subject) and a positive aspect (the object). But care is needed; for, in fact, the division subject/object is not a simple negative/positive division. If it were, only the positive would be present (as an existing phenomenon) and the negative (the subject) would not be present at all—it would simply not exist. But the subject is, in a sense, phenomenal: it (or he) is an existing phenomenal negative, a negative that appears; for the puthujjana asserts the present reality of his ‘self’ (‘the irreplaceable being that I am’). The fact is, that the intention or determination ‘mine’, pointing to a subject, is a complex structure involving avijjā.
For what it is worth, I believe that the seen that is not only the seen and the heard that is not only the heard, etc… are imagination, discursive thought, and perception. These are all embellishments on top of the sensed, the known that is only the known.
Start with the eyes. Fix your visual focus on a point and relax into a wide-angle field vision encompassing the entire visual field. Then, relax and expand your awareness to include only the heard, and eventually include all of only the felt. There won’t be any processing power left for embellishments. Eventually, depth perception and proprioception collapse. Objects in space, including the body, merge into the undifferentiated background to achieve unity.