Why did the buddha teach the mahayana?

Why did the buddha teach the mahayana?

Because Buddha knows that there are ready students for the Mahayana; same as there are ready students for the Theravada; same for Hinayana. As A BUDDHA, HE is able to teach any truth/doctrine. The Four Noble Truths is one of the fundamental teachings in Mahayana, it's not exclusive for Theravada only. If you read the Lotus Sutra (translated by Kumarajiva into Chinese), a chapter told that this Lotus Sutra was taught in the previous-previous sermon by the previous-previous Buddha. Buddha Prabhutaratna (many-treasures), the Buddha who had entered Nirvana eons of kalpas ago, appeared, and sat together with Buddha Shakyamuni inside the Prabhutaratna Stupa for this sermon of the Lotus Sutra...

Another chapter used a metaphor, to "lure" the sons leaving the burning house, the rich father said there were vehicles adorned with treasures drawn by cows, deers and etc waiting at the entrance. In fact, the father would like to give all the sons the great vehicles with most of the treasures because he was very very rich and loving.

Since all beings are the buddha-to-be; and was in essence a buddha, thus maybe you would give teaching in the time-to-be, it would be the same truth but told in the way that dictated by your audiences ;)

I completely what if answer. What if the 'mind stream' only truly makes sense when someone says that svabhava exists. And the mahayana is to realign that into 'buddha nature', awareness etc..

So the Buddha had to teach non mahayana doctrine so that sentient beings could recognize what is later revealed as lacking substance.

Buddha did not teach either Mahayana nor Theravada. There is no Theravada Canon, it is the Pali Canon, and it is canonical for all Buddhists. However, the difference is that Mahayana treats later Sutras as canon, Theravada treats commentary and Abhidhamma as canonical, differently than in other schools. The difference between the two is which apocryphal literature is accepted.

There is a significant difference between a Buddha and an Arahant. Arahant can still carry wrong views from their past due to ignorance. This can happen with the equivalent Bodhisattvas-to-be on the Mahayana path as well. A Buddha has comprehensive knowledge and right view.

So I encourage you to explore basic doctrine and see which makes the most sense to you, and follow that. A teacher and Sangha might help too.

Theravada teaches the path to individual enlightenment. Mahayana teaches the path to save all sentient beings, as follows:

"Not until the hells are emptied will I become a Buddha; not until all beings are saved will I certify to Bodhi."

"Beings are numberless, I vow to save them. Desires are inexhaustible, I vow to end them. Dharma gates are boundless, I vow to enter them. Buddha's way is unsurpassable, I vow to become it."

Bodhisattva Vows

No person has ever saved all sentient beings or emptied the hells, including any Buddhas. It would be expected a Buddha would teach what is possible & achievable. A person that has not completed their Path cannot be a Buddha.

I know that the standard Mahayana claim about the Theravada canon

I'm not sure that it's "about the Theravada canon". I don't know what you're referring to but if you look at The Jewel Ornament Of Liberation, for example, the introduction (Part 1) identifies five "families":

1. Disconnected -- no concern for what others think, no modesty, no compassion, and so forth
2. Indefinite -- might join the Hearer or Solitary Realizer or a Mahayana, depending on what kind of master they meet or what texts they study

3. Hearer -- The family of Hearers consists of those who fear samsara and yearn to achieve nirvana, but who have little compassion. It has been said:

   One who is afraid upon seeing the suffering of samsara
   And yearns to achieve nirvana
   But has little interest in benefitting sentient beings—
   These three are the marks of the Hearer family

4. Solitary Realizer -- similar to hearer except also,

   Little compassion, arrogance,
   Secretive about their teachers, and enjoying solitude—
   A wise one should understand that these are the marks of the Solitary Realizer family

5. Mahayana family

I think it's worth noting that the Mahayana is not being contrasted here with Theravada.

One tradition associated with Bodhisattva vows teaches that disparaging others is a downfall.

It is very risky for Mahayana practitioners to think that the Mahayana teachings are the highest, the most complete or advanced teachings and that the other teachings such as those of the Theravadin are lower or incomplete. Having such a feeling is really a very heavy negativity. When the Buddha taught these different liberation vehicles he did not teach that the Individual Liberation vehicle is the starting point, the Mahayana vehicle is in the middle and Vajrayana is the completion. He taught according to people’s different mental dispositions and we must understand it in that context. No one system is ‘better’, ‘higher’ or ‘more advanced’ than the others. Of course, in order to understand teachings such as the Vajrayana, an understanding of the Bodhisattva teachings on emptiness and bodhicitta is needed because there is a strong link between them and the entire Vajrayana practice is designed to make the Bodhisattva path quicker. It is very important not to have a notion that some teachings are lower or inferior to others.

Perhaps it's less to do with what the canons are, and more to do with how you relate with other people.

But do all Buddhas have to teach the four noble truths, etc., as they appear in Theravada?

The only thing I've read on that subject (i.e. about the teaching of "all Buddhas") are these verses of the Dhammapada:

On one occasion, Thera Ananda asked the Buddha whether the Fundamental Instructions to bhikkhus given by the preceding Buddhas were the same as those of the Buddha himself. To him the Buddha replied that the instructions given by all the Buddhas are as given in the following verses:

Then the Buddha spoke in verse as follows:

Verse 183: Not to do evil, to cultivate merit, to purify one's mind - this is the Teaching of the Buddhas.

Verse 184: The best moral practice is patience and forbearance; "Nibbana is Supreme", said the Buddhas. A bhikkhu does not harm others; one who harms others is not a bhikkhu.

Verse 185: Not to revile, not to do any harm, to practise restraint according to the Fundamental Instructions for the bhikkhus, to be moderate in taking food, to dwell in a secluded place, to devote oneself to higher concentration - this is the Teaching of the Buddhas.

I personally see the four noble truths as central; but I think this question showed that they're not a doctrine that make sense to everyone, and the answers to that question show that there are different aspects of Buddhism that different people consider central.

This set of lists suggests that even only within the Theravada canon, the doctrine is more than the four noble truths (or it's sometimes summarized as doctrine simpler than the four noble truths).

"I know that the standard Mahayana claim about the Theravada canon is that he needed to teach it for those who were not ready to accept the Mahayana. "

This is a skillful means explanation to a very long issues spanning centuries.

First of all you need to be aware of the Abhidhamma:

The Abhidhamma Pitaka is a detailed scholastic reworking of material appearing in the Suttas, according to schematic classifications.

The narrative:

Tradition[4] holds that the Buddha thought out the Abhidhamma immediately after his enlightenment then taught it to the gods some years later.

The facts

Scholars, however, generally date the Abhidhamma works to originating some time around the third century BCE, 100 to 200 years after the death of the Buddha. Therefore, the seven Abhidhamma works are generally claimed by scholars not to represent the words of the Buddha himself, but those of disciples and scholars

The debate

As the last major division of the canon, the Abhidhamma Pitaka has had a checkered history. It was not accepted as canonical by the Mahasanghika school[dubious – discuss][2][10] and several other schools[dubious – discuss].[11] Another school included most of the Khuddaka Nikaya within the Abhidhamma Pitaka.[2] Also, the Pali version of the Abhidhamma is a strictly Theravada collection, and has little in common with the Abhidhamma works recognized by other Buddhist schools.[12] The various Abhidhamma philosophies of the various early schools have no agreement on doctrine

The Early Mahayana sutras

Several scholars have suggested that the Prajñāpāramitā sūtras, which are among the earliest Mahāyāna sūtras,[17][18] developed among the Mahāsāṃghika along the Kṛṣṇa River in the Āndhra region of southern India.[19]

The earliest Mahāyāna sūtras to include the very first versions of the Prajñāpāramitā genre, along with texts concerning Akṣobhya Buddha, which were probably written down in the 1st century BCE in the south of India.[20][21] Guang Xing states, "Several scholars have suggested that the Prajñāpāramitā probably developed among the Mahāsāṃghikas in southern India, in the Āndhra country, on the Kṛṣṇa River."[19] A.K. Warder believes that "the Mahāyāna originated in the south of India and almost certainly in the Āndhra country."[22]

Prajñāpāramitā

Prajñāpāramitā is a central concept in Mahāyāna Buddhism and is generally associated with the doctrine of emptiness (Shunyata) or 'lack of Svabhava' (essence) and the works of Nagarjuna. Its practice and understanding are taken to be indispensable elements of the Bodhisattva path.

So why is all this important to the question?

It is because the Mahayana started out to counter the ontological claims presented in the Abhidhamma of various early Buddhist schools. You first need to understand the fundamental of the teachings before you can point out the issues within the teachings.

Many people including scholars and practitioners find Prajñāpāramitā sutras to be incomprehensible, even labeling it crazy wisdom.

However it is ultimately the application of dependent origination even to the attainments themselves. This verse by Nagarjuna in the Mulamadhyamaka Karika is the key to unlocking this understanding.

All dharmas that arise from causes and conditions,

I call them emptiness.

They are also provisional names,

And also the Middle Way.

The Chinese text

眾因緣生法，我説即是空；

亦為是假名，亦是中道義。

What Nagarjuna is saying, is that because all dependent originated phenomenon are due to causes and conditions, by their very conditionality they are impermanent and without self. As stated by the Buddha.

Pratītyasamutpāda

"When this is, that is;

This arising, that arises;

When this is not, that is not;

This ceasing, that ceases."

Therefore because things are impermanent and without a true lasting self. He call them 'empty'.

However the term 'emptiness' is also just a convenient label, because it does not fully explain the complexity of the phenomenon and all the causes that made up something. To say something is 'empty' is that it ultimately lacks an underlying 'essence', but does not mean that it does not exist but that it does not truly exist.

This is therefore the Middle Way between Eternalism and Annihilationism. To be annihilationist is to say nothing exists and that there are no causes for anything to occur. This isn't the definition of emptiness as used by Nagarjuna. It is because there are causes, there are outcomes, but because they are made up of causes, there are also impermanent, hence is not Eternal.

So whenever you come across a text like the following:

Form is Emptiness, Emptiness is form.

Heart Sutra

Your should consider the meaning of emptiness.

Form is dependently generated, Dependent origination is form.

Form is impermanent, Impermanence is form.

Form is without self, Without self is form.

So the question would be, why did Mahayana Buddhist Nagarjuna rephrase what is already a known Buddhist concept into 'emptiness'.

It because of what they perceived as the reification of Buddhist teachings in the Abhidhamma by schools, recall that even in modern Theravada teaches that Nibbanna is unconditioned, a deathless dimension, and that the Arahants are perfect etc.

Nagarjuna denies such permanence and eternalism.

This is why it is stated:

There is No Wisdom, and There is No Attainment Whatsoever.

Because wisdom is itself conditioned (by the practice of the Noble Eightfold Path), and therefore wisdom too is impermanent. Hence the fruit of Four Stages of Attainment (Stream Entry, Once Returner, One Returner, Arahantship are too impermanent).

It is not saying that there wisdom does not exist or the fruits of the contemplative life does not exist. They do exist, but like everything else are impermanent and not self.

Nagarjuna is breaking our attachments to the attainments within the very Dharma itself! Even Nirvana itself is impermanent! The end of suffering as is suffering is caused and conditioned and is impermanent! This is the meaning of non-duality in Mahayana Buddhism. It is not that wisdom and ignorance is the same. It is because they are both ultimately impermanent and cannot be permanently separated.

As you can see you first need to know what is there, such as the attainment of wisdom, the ending of suffering before you can come back and let go of even the attachments to those things. A person with no experience in Buddhism hearing 'emptiness' will either recoil with horror or become dangerously nihilistic increasing his own delusion. Even in the early Buddhist canon stated that the Buddha taught the results and rewards of good actions to lay people, encouraging them to be generous, and assuring them that they will reap the fruits of their actions. To more advance practitioners he cultivates their concentration and wisdom, finally pointing out that things are impermanent and not self. You need to know the basic goal of becoming enlightened before you realize there is never an end to the practice.

No person has ever saved all sentient beings or emptied the hells, including any Buddhas. It would be expected a Buddha would teach what is possible & achievable. A person that has not completed their Path cannot be a Buddha.

As pointed out, no person can ever be 'perfectly' enlightened either. The Mahayana Buddhists know their goal is not truly possible just as even the goals of the Theravada - extinction of rebirth is not truly possible.

If some or all Buddhas teach an enlightenment without Arhants, who in turn teach an enlightenment without Arhants, how this is any different to Arhantship, just under a different name?

In truth the names Bodhisattvas, Arhats, Buddhas are merely labels. But the distinction is the connotations that each term refers to. Arahant implies an end state to the practice. Bodhisattvas indicates that there are actually no end state, hence you can have an unenlightened bodhisattva as most beings are, or a Great Bodhisattva with immense practice. A 'Buddha' implies 'perfection' even as there is no real perfection.

Why did the Buddha teach the mahayana? No, He did not teach Mahayana. Nor did He teach Theravada, Sarvastivada, Pudgalavada, etc. and the 18 or so schools that splitted from His original Teaching/Dhamma after His passing away. Fortunately for us, all the different schools still share the same "core" regarding the Discourses and Discipline: 3 characteristics, 4NT, 5 Precepts, 8NP, 12DO, etc. Of course there're certain doctrinal disagreements among the schools which is expected and unavoidable but if one sticks to those pillars of teaching, one'd find out that they have a lot more in common than difference.