This samyutta is organized around questions that the Buddha left unanswered. Most of the discourses here focus on questions in a standard list of ten that were apparently the hot issues for philosophers in the Buddha's day: Is the cosmos eternal? Is it not eternal? Is it finite? Is it infinite? Is the body the same as the soul? Is the body one thing and the soul another? Does the Tathagata exist after death? Does he not exist after death? Both? Neither?
MN 72 lists the reasons why the Buddha does not take a position on any of these questions. In each case he says that such a position "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, & fever, and it does not lead to disenchantment, dispassion, cessation; to calm, direct knowledge, full awakening, Unbinding."
These reasons fall into two categories. The first concerns the present drawbacks of taking such a position: It is accompanied by suffering, distress, despair, and fever. The second category concerns the effects of such a position over time: It does not lead to awakening or Unbinding. AN 10.93 further explores the first category of reasons. MN 63 further explores the second.
Some of the discourses in this samyutta explore a third category of reasons for why the Buddha does not take a position on any of these questions: Such a position is based on attachment to and misunderstanding of the aggregates and sense media. When one sees these things for what they are, as they're actually present, the idea of forming them into any of these positions simply does not occur to one.
Of the discourses here, SN 44.1 and SN 44.10 are special cases. SN 44.1 focuses specifically on the questions that try to describe the status of the Tathagata after death, and explains that, having been released from the classification of the aggregates, the Tathagata defies description, in the same way that the sands of the river Ganges cannot be numbered, and the waters of the oceans cannot be calculated in gallons. The Commentary to this passage tries to fathom the Tathagata's infathomability, but its attempt is controversial. See the note to that sutta.
Even more controversial is SN 44.10, which addresses an issue not included in the standard list of ten undeclared questions: Is there a self? Is there no self? Many scholars have been uncomfortable with the fact that the Buddha leaves this question unanswered, believing that his statement that "all phenomena are not-self" implicitly states that there is no self. Thus they have tried to explain away the Buddha's silence on the existence or non-existence of the self, usually by pointing to the fourth of his reasons for not answering the question: his bewildered interlocutor, Vacchagotta, would have become even more bewildered. Had the Buddha been asked by someone less bewildered, these commentators say, he would have given the straight answer that there is no self. However, these commentators ignore two points. (1) The Buddha's first two reasons for not answering the questions have nothing to do with Vacchagotta. To say that there is a self, he says, would be siding with the wrong views of the eternalists. To say that there is no self would be siding with the wrong views of the annihilationists. (2) Immediately after Vacchagotta leaves, Ven. Ananda asks the Buddha to explain his silence. Had the Buddha really meant to declare that there is no self, this would have been the perfect time to do so, for bewildered people were now out of the way. But, again, he did not take that position.
One peculiarity of this approach to the Buddha's silence on this issue is that many commentators, noting the Buddha's desire not to bewilder Vacchagotta, assume somehow that their readers and listeners at present would not be bewildered by a doctrine that there is no self, and feel free to jump into the breach, stating baldly what they believe the Buddha was simply too reticent to say.
Another attempt to explain the Buddha's silence on this issue focuses on the second reason for his silence, saying that the annihilationists had laid claim to the slogan that there is no self, so — because the Buddha did not want his own doctrine of no self to be confused with theirs — he avoided their slogan. This explanation, however, is not supported by the Canon. The doctrines of the annihilationists are presented in a fair amount of detail in the Canon, and nowhere are they quoted as saying outright that there is no self. Thus there is no basis for saying that it was their slogan. Second, there are many instances where the Buddha, when asked a categorical question concerning an issue where he wanted to give a nuanced answer, showed himself perfectly capable of rephrasing the question in more nuanced terms before giving his reply. Had he held a nuanced doctrine that there is no self, he could have easily rephrased Vacchagotta's question before answering it. The fact that he chose not to do so, either in Vacchagotta's or Ven. Ananda's presence, indicates that he felt that this issue, too, was a thicket of views based on a misunderstanding, accompanied by suffering, and not leading to awakening.
So how is the statement "all phenomena are not self" to be taken? As a path to awakening. According to Dhp 279, when one sees this fact with discernment to the point of becoming disenchanted with stress, it forms the path to purity. Here the term "phenomena" covers fabricated and unfabricated phenomena. The fabricated phenomena encountered along the path include the aggregates, elements, and sense media. The unfabricated phenomenon, encountered when these fabricated phenomena cease, is the deathless. AN 9.96, however, points out that it is possible, on encountering the deathless, to feel a dhamma-passion and dhamma-delight for it, thus preventing full awakening. At this point the realization that all phenomena are not-self would be needed to overcome this last obstacle to total release. And once there is release, one becomes, like the Tathagata, indescribable: "deep, boundless, hard to fathom, like the ocean." At that point, the path is abandoned, like a raft after it has been used to cross a river, and positions that "there is a self" and "there is no self" would not apply.