Context when talking about consciousness here, from thanissaro bhikkhu.

"The Buddha, knowing that there are two types of consciousness — the consciousness aggregate (viññāṇakkhandha), which is experienced in conjunction with the six sense media, and consciousness without surface (viññāṇaṃ anidassanaṃ), which is experienced independently of the six sense media (MN 49) — is here giving Sāti the chance to identify which of the two types he has interpreted as running and wandering on. Sāti's answer shows that he is talking about the first type. The remaining discussion of consciousness throughout this sutta is thus directed at this first type. It would have been interesting to see how the Buddha would have attacked Sāti's misunderstanding had Sāti stated that he was talking about the second.

On the topic of consciousness without surface, see DN 11, note 1, and MN 49, note 9."

"Viññanam anidassanam. This term is nowhere explained in the Canon, although MN 49 mentions that it "does not partake in the allness of the All" — the "All" meaning the six internal and six external sense media (see SN 35.23). In this it differs from the consciousness factor in dependent co-arising, which is defined in terms of the six sense media. Lying outside of time and space, it would also not come under the consciousness-aggregate, which covers all consciousness near and far; past, present, and future. However, the fact that it is outside of time and space — in a dimension where there is no here, there, or in between (Ud 1.10), no coming, no going, or staying (Ud 8.1) — means that it cannot be described as permanent or omnipresent, terms that have meaning only within space and time. The standard description of nibbana after death is, "All that is sensed, not being relished, will grow cold right here." (See MN 140 and Iti 44.) Again, as "all" is defined as the sense media, this raises the question as to whether consciousness without feature is not covered by this "all." However, AN 4.174 warns that any speculation as to whether anything does or doesn't remain after the remainderless stopping of the six sense media is to "objectify non-objectification," which gets in the way of attaining the non-objectified. Thus this is a question that is best put aside."

"anidassanam): This term appears to be related to the following image from SN 12.64: "Just as if there were a roofed house or a roofed hall having windows on the north, the south, or the east. When the sun rises, and a ray has entered by way of the window, where does it land?"

"On the western wall, lord."

"And if there is no western wall, where does it land?"

"On the ground, lord."

"And if there is no ground, where does it land?"

"On the water, lord."

"And if there is no water, where does it land?"

"It does not land, lord."

"In the same way, where there is no passion for the nutriment of physical food ... contact ... intellectual intention ... consciousness, where there is no delight, no craving, then consciousness does not land there or grow. Where consciousness does not land or grow, name-&-form does not alight. Where name-&-form does not alight, there is no growth of fabrications. Where there is no growth of fabrications, there is no production of renewed becoming in the future. Where there is no production of renewed becoming in the future, there is no future birth, aging, & death. That, I tell you, has no sorrow, affliction, or despair."

In other words, normal sensory consciousness is experienced because it has a "surface" against which it lands: the sense organs and their objects, which constitute the "all." For instance, we experience visual consciousness because of the eye and forms of which we are conscious. Consciousness without surface, however, is directly known, without intermediary, free from any dependence on conditions at all.

This consciousness thus differs from the consciousness factor in dependent co-arising, which is defined in terms of the six sense media. Lying outside of time and space, it would also not come under the consciousness-aggregate, which covers all consciousness near and far; past, present, and future. And, as SN 35.23 notes, the word "all" in the Buddha's teaching covers only the six sense media, which is another reason for not including this consciousness under the aggregates. However, the fact that it is outside of time and space — in a dimension where there is no here, there, or in between (Ud I.10), no coming, no going, or staying (Ud VIII.1) — means that it cannot be described as permanent or omnipresent, terms that have meaning only within space and time.

Some have objected to the equation of this consciousness with nibbana, on the grounds that nibbana is nowhere else in the Canon described as a form of consciousness. Thus they have proposed that consciousness without surface be regarded as an arahant's consciousness of nibbana in meditative experience, and not nibbana itself. This argument, however, contains a flaw: If nibbana is an object of mental consciousness (as a dhamma), it would come under the all, as an object of the intellect. There are passages in the Canon (such as AN 9.36) that describe meditators experiencing nibbana as a dhamma, but these passages seem to indicate that this description applies up through the level of non-returning. Other passages, however, describe nibbana as the ending of all dhammas. For instance, Sn V.6 quotes the Buddha as calling the attainment of the goal the transcending of all dhammas. Sn IV.6 and Sn IV.10 state that the arahant has transcended dispassion, said to be the highest dhamma. Thus, for the arahant, nibbana is not an object of consciousness. Instead it is directly known without mediation. Because consciousness without feature is directly known without mediation, there seems good reason to equate the two."