user-1: 
Was rebirth introduced in the First or Second Turnings, and why did the Buddha teach it? E.g. was it necessary to understand emptiness, causality, etc.? 
For example, I imagine that some "reasons why" might include:

Maybe it helps deemphasize a person's current lifetime, highlighting its impermanence, so that one does not cling to it or themselves.
On the other hand, for some, it seems to be a source of inspiration to persevere on the Path, for a "good rebirth".

Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?


user-1: The Buddha taught karma and rebirth under mundane right view -

Right view, I say, is twofold: there is right view that is affected by influxes, partaking of merit, ripening in the acquisitions; and there is right view that is noble, free of influxes, supramundane, a factor of the path.
AN 6.63

and as Bhikku Bodhi goes on to write in The Buddha's Teachings on Social and Communal Harmony

“Right view affected by influxes” implies that even as we attempt to see and act in accord with the Dharma, we are still affected by the delusion of self. “Partaking of merit” is using Buddhist practice for what we perceive as our own benefit. “Ripening in the acquisitions” means becoming or acquiring a self. These are mundane or worldly ways.

Since this mundane right view is still "affected by influxes", with notions of self, not only is rebirth unnecessary to understand not-self, it may in fact be incompatible, as further explained in this answer.
This may be why the Buddha taught  superior right view. It is this right view which leads to liberation, as Bhikku Bodhi writes -

This superior right view leading to liberation is the understanding of the Four Noble Truths. It is this right view that figures as the first factor of the Noble Eightfold Path in the proper sense: as the noble right view. Thus the Buddha defines the path factor of right view expressly in terms of the four truths: “What now is right view? It is understanding of suffering (dukkha), understanding of the origin of suffering, understanding of the cessation of suffering, understanding of the way leading to the cessation of suffering

Belief in rebirth (as reincarnation) is also unnecessary in regards to ethics and morality, as Bhikku Bodhi further writes -

Several texts testify that the Buddha himself seems to have recognized that morality can be established on the basis of self- reflection and ethical reasoning without requiring a belief in personal survival of death.


user13726: I do not think the Buddha taught that it was necessary to assume rebirth.
Take this from the Kalama Sutta:

"'Suppose there is a hereafter and there is a fruit, result, of deeds
  done well or ill. Then it is possible that at the dissolution of the
  body after death, I shall arise in the heavenly world, which is
  possessed of the state of bliss.' This is the first solace found by
  him.
"'Suppose there is no hereafter and there is no fruit, no result, of
  deeds done well or ill. Yet in this world, here and now, free from
  hatred, free from malice, safe and sound, and happy, I keep myself.'
  This is the second solace found by him.

Clearly, any of the meditative practices the Buddha taught focus attention on the rebirth that happens in this moment, here and now.

user254: Was rebirth introduced in the First or Second Turnings, and why did the Buddha teach it?
I think the idea of rebirth preceded (predated, already existed earlier than) the Buddha's doctrine:

Early Hinduism, Jainism and Buddhism
The idea of reincarnation has early roots in the Vedic period (c. 1500 – c. 500 BCE), predating the Buddha and the Mahavira.

I think that, therefore, the question[s] which the Buddhist doctrine had to answer included, "Is the 'rebirth' doctrine true, to what extent is it true (how does it fit with other Buddhist doctrines), what does it mean exactly, how does it work, what is kamma?" and similar questions.
Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?
I don't know, maybe some of these might be affected:

Stages of enlightenment -- that perhaps you won't be enlightened in this life, but even so that's not too late (and that effort is always even though immediate result may seem impossible)
An incentive (albeit "siding with merit and resulting in acquisition") for morality: including, both, generosity and harmlessness
The concept of merit may affect the relationship between lay society and the sangha
The three characteristics:

anatta e.g. as explained in this answer -- if rebirth doesn't exist, does that mean that we live and die, and if so isn't that identifying with the aggregates?
anicca -- so you're saying that death is permanent, not impermanent?
dukkha -- without the doctrine of rebirth, some people may develop strange or wrong ideas around killing or suicide as being an escape from dukkha ... also the doctrine that "craving to live (or, for becoming)" and "craving to die (or, for cessation)" are, both, forms of craving

The superiority (super-humanity) of the Buddha (not according to the suttas, necessarily, but e.g. the Jataka tales and the Mahāvaṃsa)
Combining cyclic cosmology with the dhamma's being timeless (previous and future Buddhas)
Atheism -- no sense in worshipping the Gods too much, they too are subject to rebirth
Hope or optimism -- no matter how bad things seem now, etc.
Some sense in which dhamma is called timeless (akalika)


user-1: The concept of rebirth is an axiomatic corolary of 'Karma'.
Also if rebirth does not exist then nibbana i.e. 'blowing out' doesn't make sense.
For e.g. without rebirth and without afterlife in heaven or hell you will be  essentially 'blown out' nibbanafied without doing anything.
So nibbana makes sense only because rebirth exist, because in rebirth the flame continues.
Also, murdering sentient beings (in a painless way) would also be equivalent to liberating them from suffering if rebirth does not exist.
So it is a pretty central doctrine, even if you don't emphasize on Karma.

user471: 
OP: Was rebirth introduced in the First or Second Turnings?

Rebirth has been around since the Earth first started turning on its axis. And perhaps much.. much.. longer than that.
The Buddha stated in the Assu Sutta:

"This is the greater: the tears you have shed while transmigrating &
  wandering this long, long time — crying & weeping from being joined
  with what is displeasing, being separated from what is pleasing — not
  the water in the four great oceans.
"Long have you (repeatedly) experienced the death of a mother. The
  tears you have shed over the death of a mother while transmigrating &
  wandering this long, long time — crying & weeping from being joined
  with what is displeasing, being separated from what is pleasing — are
  greater than the water in the four great oceans.
"Long have you (repeatedly) experienced the death of a father... the
  death of a brother... the death of a sister... the death of a son...
  the death of a daughter... loss with regard to relatives... loss with
  regard to wealth... loss with regard to disease. The tears you have
  shed over loss with regard to disease while transmigrating & wandering
  this long, long time — crying & weeping from being joined with what is
  displeasing, being separated from what is pleasing — are greater than
  the water in the four great oceans.
"Why is that? From an inconstruable beginning comes transmigration.
  A beginning point is not evident, though beings hindered by ignorance
  and fettered by craving are transmigrating & wandering on. Long have
  you thus experienced stress, experienced pain, experienced loss,
  swelling the cemeteries — enough to become disenchanted with all
  fabricated things, enough to become dispassionate, enough to be
  released."

Based on the sutta above, I made a calculation in this answer that every person has been reborn at least 20 quintillion (20 x 1018) times. But I'm guessing that it's a lot longer than that.

OP: why did the Buddha teach it? E.g. was it necessary to understand emptiness, causality, etc.?

The Buddha taught it because this was what he realized to be true, and this knowledge is also useful to understand two things. The first is that suffering has happened since forever. The second is that it is not possible to escape suffering through death. Only ending craving and the ten fetters would result in freedom from suffering.
According to the Dvedhavitakka Sutta:

"When the mind was thus concentrated, purified, bright, unblemished,
  rid of defilement, pliant, malleable, steady, & attained to
  imperturbability, I directed it to the knowledge of recollecting my
  past lives. I recollected my manifold past lives, i.e., one birth,
  two... five, ten... fifty, a hundred, a thousand, a hundred thousand,
  many eons of cosmic contraction, many eons of cosmic expansion, many
  eons of cosmic contraction & expansion: 'There I had such a name,
  belonged to such a clan, had such an appearance. Such was my food,
  such my experience of pleasure & pain, such the end of my life.
  Passing away from that state, I re-arose there. There too I had such a
  name, belonged to such a clan, had such an appearance. Such was my
  food, such my experience of pleasure & pain, such the end of my life.
  Passing away from that state, I re-arose here.' Thus I remembered my
  manifold past lives in their modes & details.
"This was the first knowledge I attained in the first watch of the
  night. Ignorance was destroyed; knowledge arose; darkness was
  destroyed; light arose — as happens in one who is heedful, ardent, &
  resolute.
"When the mind was thus concentrated, purified, bright, unblemished,
  rid of defilement, pliant, malleable, steady, & attained to
  imperturbability, I directed it to the knowledge of the passing away &
  reappearance of beings. I saw — by means of the divine eye, purified
  & surpassing the human — beings passing away & re-appearing, and I
  discerned how they are inferior & superior, beautiful & ugly,
  fortunate & unfortunate in accordance with their kamma: 'These beings
  — who were endowed with bad conduct of body, speech & mind, who
  reviled the Noble Ones, held wrong views and undertook actions under
  the influence of wrong views — with the break-up of the body, after
  death, have re-appeared in the plane of deprivation, the bad
  destination, the lower realms, in hell. But these beings — who were
  endowed with good conduct of body, speech, & mind, who did not revile
  the Noble Ones, who held right views and undertook actions under the
  influence of right views — with the break-up of the body, after
  death, have re-appeared in the good destinations, in the heavenly
  world.' Thus — by means of the divine eye, purified & surpassing the
  human — I saw beings passing away & re-appearing, and I discerned how
  they are inferior & superior, beautiful & ugly, fortunate &
  unfortunate in accordance with their kamma.

Since the Buddha made speaking the truth a very important precept, it is obvious that he couldn't be lying when he said that he saw with his divine eye, the truth of rebirth, where, with the break-up of the body, after death, beings reappear in other destinations. The phrase "break-up of the body" shows that it is not a metaphorical death.

OP: Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?

If it is possible to escape suffering through death, the Buddha would have taught suicide. Basically, the Noble Eightfold Path becomes redundant without rebirth.
Rebirth is actually not the important point. The important point is that it is not possible to become free from suffering through death.

user13375: The Truth of Rebirth seems to be one of the larger points of dispute between participants on this site. See here and here for similar questions and answers. I guess this is not all that surprising to me since the site seems heavily represented by Western practitioners. In my experience it seems that westerners have (and I definitely include myself here) a particularly difficult time with the Truth of Rebirth as taught by the Buddha. Here is what Venerable Thanissaro Bhikkhu has to say about this:

Yet as these Buddhist religions have come to the West, they have run
into a barrier from modern Western culture: Of all the Buddha's
teachings, rebirth has been one of the hardest for modern Westerners
to accept. Part of this resistance comes from the fact that none of
the dominant world-views of Western culture, religious or
materialistic, contain anything corresponding to the idea of repeated
rebirth. Plato taught it, but — aside from an esoteric fringe — few in
the modern West have treated this side of his teaching as anything
more than a myth.
For people who have felt burned or repelled by the faith demands of
Western religion, there is the added barrier that the teaching on
rebirth is something that — for the unawakened — has to be taken on
faith. They would prefer a Buddhism that makes no faith demands,
focusing its attention solely on the benefits it can bring in this
life.

There seem to be many approaches on this site:

Simply denying outright that the Buddha taught rebirth.
The Buddha taught the reality of rebirth as a (re)birth that takes place in this very life whenever self-views of "I" and "mine" occur. No "afterlife" concept of rebirth was taught or acknowledged by the Buddha.
Admitting he taught it, but insisting it was just skillful means.
Rebirth as a combination of metaphor and upaya.
Those who take rebirth at face value, but insist that mere experience can end.
Like above, but do not posit an end to this mere experience.
Those who think rebirth (generally) can be known incontrovertibly through reason alone.
Those who think rebirth requires a dose of faith to be known (generally).

And probably a number of other views that I've missed. Myself, I'm with 6 and 7.
Was rebirth introduced in the First or Second Turnings, and why did the Buddha teach it?
To my mind, unquestionably it was taught and introduced. I think the Buddha taught it because it was/is a conventional truth and the Buddha taught the truth.
Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?
Yes, I think there are obvious logical problems and inconsistencies that present if you deny the truth of rebirth. For instance, as BodhiWalker mentions above one could seek the end of suffering by merely ending this current life. Even more problematic, one could "liberate" others from suffering by murdering sentient beings painlessly. That's monstrous and I want nothing to do with it. I want to make quite clear, I'm sure this is not what others have in mind when denying or glossing the truth of rebirth. I'm emphatically not concluding that those who deny or gloss the truth of rebirth are murderous monsters, just that denying or glossing the truth of rebirth presents logical problems that I don't see a way around.
Anyway, I think Venerable Thanissaro Bhikkhu's article above is excellent and highly recommend reading it many times and contemplating these questions.

-1: <ol>
<li><p>Was rebirth introduced in the First or Second Turnings, and why did the Buddha teach it? E.g. was it necessary to understand emptiness, causality, etc.? 
For example, I imagine that some "reasons why" might include:</p>

<ul>
<li>Maybe it helps deemphasize a person's current lifetime, highlighting its impermanence, so that one does not cling to it or themselves.</li>
<li>On the other hand, for some, it seems to be a source of inspiration to persevere on the Path, for a "good rebirth".</li>
</ul></li>
<li><p>Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?</p></li>
</ol>
 -1: <p>The Buddha taught karma and rebirth under <em>mundane right view</em> -</p>
<blockquote>
<p><em>Right view, I say, is twofold: there is right view that is affected by influxes, partaking of merit, ripening in the acquisitions; and there is right view that is noble, free of influxes, supramundane, a factor of the path.</em></p>
<p><a href="http://www.accesstoinsight.org/tipitaka/an/an06/an06.063.than.html" rel="nofollow noreferrer">AN 6.63</a></p>
</blockquote>
<p>and as Bhikku Bodhi goes on to write in <a href="https://rads.stackoverflow.com/amzn/click/com/B01G2DD5QY" rel="nofollow noreferrer" rel="nofollow noreferrer">The Buddha's Teachings on Social and Communal Harmony</a></p>
<blockquote>
<p>“Right view affected by influxes” implies that even as we attempt to see and act in accord with the Dharma, we are still affected by the <strong>delusion of self</strong>. “Partaking of merit” is using Buddhist practice for what we perceive as our own benefit. “Ripening in the acquisitions” means becoming or <strong>acquiring a self</strong>. These are mundane or worldly ways.</p>
</blockquote>
<p>Since this <em>mundane right view</em> is still &quot;affected by influxes&quot;, with notions of self, not only is rebirth unnecessary to understand not-self, it may in fact be incompatible, as further explained in <a href="https://buddhism.stackexchange.com/a/19773/8619">this answer</a>.</p>
<p>This may be why the Buddha taught  <em>superior right view</em>. It is this right view which leads to liberation, as <a href="https://rads.stackoverflow.com/amzn/click/com/B007626M3E" rel="nofollow noreferrer" rel="nofollow noreferrer">Bhikku Bodhi writes</a> -</p>
<blockquote>
<p>This <strong>superior right view</strong> leading to liberation is the understanding of the Four Noble Truths. It is this right view that figures as the first factor of the Noble Eightfold Path in the proper sense: as the noble right view. Thus the Buddha defines the path factor of right view expressly in terms of the four truths: “What now is right view? It is understanding of suffering (dukkha), understanding of the origin of suffering, understanding of the cessation of suffering, understanding of the way leading to the cessation of suffering</p>
</blockquote>
<p>Belief in rebirth (as reincarnation) is also unnecessary in regards to ethics and morality, as <a href="https://rads.stackoverflow.com/amzn/click/com/B01G2DD5QY" rel="nofollow noreferrer" rel="nofollow noreferrer">Bhikku Bodhi further writes</a> -</p>
<blockquote>
<p>Several texts testify that the Buddha himself seems to have recognized that morality can be established on the basis of self- reflection and ethical reasoning without requiring a belief in personal survival of death.</p>
</blockquote>
 13726: <p>I do not think the Buddha taught that it was necessary to assume rebirth.
Take this from the Kalama Sutta:</p>

<blockquote>
  <p>"'Suppose there is a hereafter and there is a fruit, result, of deeds
  done well or ill. Then it is possible that at the dissolution of the
  body after death, I shall arise in the heavenly world, which is
  possessed of the state of bliss.' This is the first solace found by
  him.</p>
  
  <p>"'Suppose there is no hereafter and there is no fruit, no result, of
  deeds done well or ill. Yet in this world, here and now, free from
  hatred, free from malice, safe and sound, and happy, I keep myself.'
  This is the second solace found by him.</p>
</blockquote>

<p>Clearly, any of the meditative practices the Buddha taught focus attention on the rebirth that happens in this moment, here and now.</p>
 254: <p><strong>Was rebirth introduced in the First or Second Turnings, and why did the Buddha teach it?</strong></p>

<p>I think the idea of rebirth preceded (predated, already existed earlier than) the Buddha's doctrine:</p>

<blockquote>
  <p><a href="https://en.wikipedia.org/wiki/Reincarnation#Early_Hinduism,_Jainism_and_Buddhism" rel="nofollow noreferrer">Early Hinduism, Jainism and Buddhism</a></p>
  
  <p>The idea of reincarnation has early roots in the Vedic period (c. 1500 – c. 500 BCE), predating the Buddha and the Mahavira.</p>
</blockquote>

<p>I think that, therefore, the question[s] which the Buddhist doctrine had to answer included, "Is the 'rebirth' doctrine true, to what extent is it true (how does it fit with other Buddhist doctrines), what does it mean exactly, how does it work, what is kamma?" and similar questions.</p>

<p><strong>Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?</strong></p>

<p>I don't know, maybe some of these might be affected:</p>

<ul>
<li><a href="https://en.wikipedia.org/wiki/Four_stages_of_enlightenment" rel="nofollow noreferrer">Stages of enlightenment</a> -- that perhaps you won't be enlightened in this life, but even so that's not too late (and that effort is always even though immediate result may seem impossible)</li>
<li>An incentive (albeit "siding with merit and resulting in acquisition") for morality: including, both, generosity and harmlessness</li>
<li>The concept of <a href="https://en.wikipedia.org/wiki/Merit_(Buddhism)" rel="nofollow noreferrer">merit</a> may affect the relationship between lay society and the sangha</li>
<li><p>The three characteristics:</p>

<ul>
<li><em>anatta</em> e.g. as explained in <a href="https://buddhism.stackexchange.com/a/26364/254">this answer</a> -- if rebirth doesn't exist, does that mean that we live and die, and if so isn't that identifying with the aggregates?</li>
<li><em>anicca</em> -- so you're saying that death is permanent, not impermanent?</li>
<li><em>dukkha</em> -- without the doctrine of rebirth, some people may develop strange or wrong ideas around killing or suicide as being an escape from dukkha ... also the doctrine that "craving to live (or, for becoming)" and "craving to die (or, for cessation)" are, both, forms of craving</li>
</ul></li>
<li><p>The superiority (super-humanity) of the Buddha (not according to the suttas, necessarily, but e.g. the Jataka tales and the Mahāvaṃsa)</p></li>
<li>Combining cyclic cosmology with the dhamma's being timeless (previous and future Buddhas)</li>
<li>Atheism -- no sense in worshipping the Gods too much, they too are subject to rebirth</li>
<li>Hope or optimism -- no matter how bad things seem now, etc.</li>
<li>Some sense in which dhamma is called timeless (<em>akalika</em>)</li>
</ul>
 -1: <p>The concept of rebirth is an axiomatic corolary of 'Karma'.</p>

<p>Also if rebirth does not exist then <em>nibbana</em> i.e. 'blowing out' doesn't make sense.</p>

<p>For e.g. without rebirth and without afterlife in heaven or hell you will be  essentially 'blown out' <em>nibbanafied</em> without doing anything.</p>

<p>So <em>nibbana</em> makes sense only because rebirth exist, because in rebirth the flame continues.</p>

<p>Also, murdering sentient beings (in a painless way) would also be equivalent to liberating them from suffering if rebirth does not exist.</p>

<p>So it is a pretty central doctrine, even if you don't emphasize on Karma.</p>
 471: <blockquote>
  <p><strong>OP:</strong> Was rebirth introduced in the First or Second Turnings?</p>
</blockquote>

<p>Rebirth has been around since the Earth first started turning on its axis. And perhaps much.. much.. longer than that.</p>

<p>The Buddha stated in the <a href="https://www.accesstoinsight.org/tipitaka/sn/sn15/sn15.003.than.html" rel="nofollow noreferrer">Assu Sutta</a>:</p>

<blockquote>
  <p>"This is the greater: the tears you have shed while transmigrating &amp;
  wandering this long, long time — crying &amp; weeping from being joined
  with what is displeasing, being separated from what is pleasing — not
  the water in the four great oceans.</p>
  
  <p>"Long have you (repeatedly) experienced the death of a mother. The
  tears you have shed over the death of a mother while transmigrating &amp;
  wandering this long, long time — crying &amp; weeping from being joined
  with what is displeasing, being separated from what is pleasing — are
  greater than the water in the four great oceans.</p>
  
  <p>"Long have you (repeatedly) experienced the death of a father... the
  death of a brother... the death of a sister... the death of a son...
  the death of a daughter... loss with regard to relatives... loss with
  regard to wealth... loss with regard to disease. The tears you have
  shed over loss with regard to disease while transmigrating &amp; wandering
  this long, long time — crying &amp; weeping from being joined with what is
  displeasing, being separated from what is pleasing — are greater than
  the water in the four great oceans.</p>
  
  <p>"Why is that? <strong>From an inconstruable beginning comes transmigration.
  A beginning point is not evident, though beings hindered by ignorance
  and fettered by craving are transmigrating &amp; wandering on.</strong> Long have
  you thus experienced stress, experienced pain, experienced loss,
  swelling the cemeteries — enough to become disenchanted with all
  fabricated things, enough to become dispassionate, enough to be
  released."</p>
</blockquote>

<p>Based on the sutta above, I made a calculation in <a href="https://buddhism.stackexchange.com/a/23751/471">this answer</a> that every person has been reborn at least 20 quintillion (20 x 10<sup>18</sup>) times. But I'm guessing that it's a lot longer than that.</p>

<blockquote>
  <p><strong>OP:</strong> why did the Buddha teach it? E.g. was it necessary to understand emptiness, causality, etc.?</p>
</blockquote>

<p>The Buddha taught it because this was what he realized to be true, and this knowledge is also useful to understand two things. The first is that suffering has happened since forever. The second is that it is not possible to escape suffering through death. Only ending craving and the ten fetters would result in freedom from suffering.</p>

<p>According to the <a href="https://www.accesstoinsight.org/tipitaka/mn/mn.019.than.html" rel="nofollow noreferrer">Dvedhavitakka Sutta</a>:</p>

<blockquote>
  <p>"When the mind was thus concentrated, purified, bright, unblemished,
  rid of defilement, pliant, malleable, steady, &amp; attained to
  imperturbability, <strong>I directed it to the knowledge of recollecting my
  past lives. I recollected my manifold past lives</strong>, i.e., one birth,
  two... five, ten... fifty, a hundred, a thousand, a hundred thousand,
  many eons of cosmic contraction, many eons of cosmic expansion, many
  eons of cosmic contraction &amp; expansion: 'There I had such a name,
  belonged to such a clan, had such an appearance. Such was my food,
  such my experience of pleasure &amp; pain, such the end of my life.
  Passing away from that state, I re-arose there. There too I had such a
  name, belonged to such a clan, had such an appearance. Such was my
  food, such my experience of pleasure &amp; pain, such the end of my life.
  Passing away from that state, I re-arose here.' Thus I remembered my
  manifold past lives in their modes &amp; details.</p>
  
  <p>"This was the first knowledge I attained in the first watch of the
  night. Ignorance was destroyed; knowledge arose; darkness was
  destroyed; light arose — as happens in one who is heedful, ardent, &amp;
  resolute.</p>
  
  <p>"When the mind was thus concentrated, purified, bright, unblemished,
  rid of defilement, pliant, malleable, steady, &amp; attained to
  imperturbability, I directed it to the knowledge of the passing away &amp;
  reappearance of beings. <strong>I saw — by means of the divine eye, purified
  &amp; surpassing the human — beings passing away &amp; re-appearing</strong>, and I
  discerned how they are inferior &amp; superior, beautiful &amp; ugly,
  fortunate &amp; unfortunate in accordance with their kamma: 'These beings
  — who were endowed with bad conduct of body, speech &amp; mind, who
  reviled the Noble Ones, held wrong views and undertook actions under
  the influence of wrong views — <strong>with the break-up of the body, after
  death, have re-appeared</strong> in the plane of deprivation, the bad
  destination, the lower realms, in hell. But these beings — who were
  endowed with good conduct of body, speech, &amp; mind, who did not revile
  the Noble Ones, who held right views and undertook actions under the
  influence of right views — <strong>with the break-up of the body, after
  death, have re-appeared</strong> in the good destinations, in the heavenly
  world.' Thus — by means of the divine eye, purified &amp; surpassing the
  human — I saw beings passing away &amp; re-appearing, and I discerned how
  they are inferior &amp; superior, beautiful &amp; ugly, fortunate &amp;
  unfortunate in accordance with their kamma.</p>
</blockquote>

<p>Since the Buddha made speaking the truth a very important precept, it is obvious that he couldn't be lying when he said that he saw with his divine eye, the truth of rebirth, where, with the break-up of the body, after death, beings reappear in other destinations. The phrase "break-up of the body" shows that it is not a metaphorical death.</p>

<blockquote>
  <p><strong>OP:</strong> Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?</p>
</blockquote>

<p>If it is possible to escape suffering through death, the Buddha would have taught suicide. Basically, the Noble Eightfold Path becomes redundant without rebirth.</p>

<p>Rebirth is actually not the important point. The important point is that it is not possible to become free from suffering through death.</p>
 13375: <p>The <a href="https://www.accesstoinsight.org/lib/authors/thanissaro/truth_of_rebirth.html" rel="nofollow noreferrer">Truth of Rebirth</a> seems to be one of the larger points of dispute between participants on this site. See <a href="https://buddhism.stackexchange.com/questions/10361/dharma-without-karma">here</a> and <a href="https://buddhism.stackexchange.com/questions/3420/is-rebirth-a-delusional-belief">here</a> for similar questions and answers. I guess this is not all that surprising to me since the site seems heavily represented by Western practitioners. In my experience it seems that westerners have (<em>and I definitely include myself here</em>) a particularly difficult time with the Truth of Rebirth as taught by the Buddha. Here is what <a href="https://www.accesstoinsight.org/lib/authors/thanissaro/truth_of_rebirth.html" rel="nofollow noreferrer">Venerable Thanissaro Bhikkhu has to say about this:</a></p>
<blockquote>
<p>Yet as these Buddhist religions have come to the West, they have run
into a barrier from modern Western culture: Of all the Buddha's
teachings, rebirth has been one of the hardest for modern Westerners
to accept. Part of this resistance comes from the fact that none of
the dominant world-views of Western culture, religious or
materialistic, contain anything corresponding to the idea of repeated
rebirth. Plato taught it, but — aside from an esoteric fringe — few in
the modern West have treated this side of his teaching as anything
more than a myth.</p>
<p>For people who have felt burned or repelled by the faith demands of
Western religion, there is the added barrier that the teaching on
rebirth is something that — for the unawakened — has to be taken on
faith. They would prefer a Buddhism that makes no faith demands,
focusing its attention solely on the benefits it can bring in this
life.</p>
</blockquote>
<p>There seem to be many approaches on this site:</p>
<ol>
<li>Simply denying outright that the Buddha taught rebirth.</li>
<li>The Buddha taught the reality of rebirth as a (re)birth that takes place in this very life whenever self-views of &quot;I&quot; and &quot;mine&quot; occur. No &quot;afterlife&quot; concept of rebirth was taught or acknowledged by the Buddha.</li>
<li>Admitting he taught it, but insisting it was just skillful means.</li>
<li>Rebirth as a <em>combination</em> of metaphor and <a href="https://en.wikipedia.org/wiki/Upaya" rel="nofollow noreferrer">upaya</a>.</li>
<li>Those who take rebirth at face value, but insist that mere experience can end.</li>
<li>Like above, but do not posit an end to this mere experience.</li>
<li>Those who think rebirth (generally) can be known incontrovertibly through reason alone.</li>
<li>Those who think rebirth requires a dose of faith to be known (generally).</li>
</ol>
<p>And probably a number of other views that I've missed. Myself, I'm with 6 and 7.</p>
<p><strong>Was rebirth introduced in the First or Second Turnings, and why did the Buddha teach it?</strong></p>
<p>To my mind, <em>unquestionably</em> it was taught and introduced. I think the Buddha taught it because it was/is a conventional truth and the Buddha taught the truth.</p>
<p><strong>Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?</strong></p>
<p>Yes, I think there are obvious logical problems and inconsistencies that present if you deny the truth of rebirth. For instance, as BodhiWalker mentions above one could seek the end of suffering by merely ending this current life. Even more problematic, one could &quot;liberate&quot; others from suffering by murdering sentient beings painlessly. That's monstrous and I want nothing to do with it. I want to make quite clear, I'm sure <em>this is not what others have in mind</em> when denying or glossing the truth of rebirth. I'm emphatically not concluding that those who deny or gloss the truth of rebirth are murderous monsters, just that denying or glossing the truth of rebirth presents logical problems that I don't see a way around.</p>
<p>Anyway, I think Venerable Thanissaro Bhikkhu's article above is excellent and highly recommend reading it many times and contemplating these questions.</p>
