How do Mahayana Buddhists perceive the world?

This means that every Mahayana Buddhist believes that he lives in a world in which all people do not have consciousness, sensation and emotion and he thinks that all other people are just a projection of his mind, but he treats them as conscious sentient beings?

This is incorrect and the answer on Quora to this question is not representative. The Mahayana is a broad vehicle and followers of the Mahayana are at many different stages of the path. Beginners in the Mahayana path naturally have much different beliefs than those who are approaching the perfection of the path. The very question itself is much too simplistic to get any kind of rewarding or satisfying answer.

If you have a specific question about a specific doctrine/person of the very many who belong to or ascribe to the Mahayana path, then please go ahead and ask such a question. However, I would advise that a more efficient way to get a glimpse into what Mahayana believes is to pick a specific school and study. You could also look at numerous beginner books. This is the one I would suggest. That should give you an overview of the various schools in Buddhism and what they generally believe and where they disagree.

From my current understanding this is how I see it.

Firstly, I need to explain the average run-of-the-mill mind...

In Mahayana what defines a person is ego. In its most simplistic terms ego is a collection of thoughts and feelings that cause one to lurch aimlessly towards various degrees of pleasure and retract from various degrees of pain. It is a reactive measure that aims to create a sense of safety and security. These very acts of aversion and attraction give the sense that we are in control, that the decisions we are making are individualized, personal and contribute to our wellbeing.

The truth is that all those feelings, thoughts, aversions and attractions centre around the characterization of worldly perceptions shrunk into a singularity called [insert name here]. That is to say, they have arisen in dependence upon whatever has been perceived, but then mistaken itself as existing in conflict to the world it lives within; the very world ego gathered its characterization from. By isolating oneself from the world in this way, suffering endures. This is called ignorance or to use a more user-friendly term not seeing clearly.

With the above in mind, when we act as if each person has his or her own consciousness, sensation, and thoughts, there are two things happening...

1. That they are actually already free from ego and suffering. They have Buddha nature right now. Sunyata.

2. But yet they are ignorant: not seeing what is right before them. The recognition of this is a recognition of an independent consciousness or ignorance. You cannot ignore ignorance or else there would be no compassion, and thus you couldn't give a teaching or help in other ways.

Just to make this somewhat paradoxical point more prominent, chapter 17 of the Diamond Sutra might help neutralize the dissonance that occurs from that paradox, but understand that the teaching is the recognition of ignorance and where it points is sunyata; they are seemingly one and the same.

“Subhuti, it is just the same when a disciple speaks of liberating numberless sentient beings. If they have in mind any arbitrary conception of sentient beings or of definite numbers, then they are unworthy of being called a disciple. Subhuti, my teachings reveal that even such a thing as is called a ‘disciple’ is non-existent. Furthermore, there is really nothing for a disciple to liberate.”

“A true disciple knows that there is no such thing as a self, a person, a living being, or a universal self. A true disciple knows that all things are devoid of selfhood, devoid of any separate individuality.”

This moves us nicely onto your last point: it is not that Mahayana believes that all is a projection of mind, but that all perceptions operate in unison with the world and that there cannot be personal decisions made on the basis of the incalculable processes currently at play. There is no special character in the middle of it all. All is what is.

If you had said, "but I made a decision to upload this question". That is absurd! Think about all the necessary requirements that were needed for you to type and upload this question: the computer and it's components, electricity, internet, your mother and father... You made no decision, it just happened in dependence on other factors.

In this way, sunyata is how we come to understand the emptiness of how we erroneously characterize ourselves, and we see the sheer complexity and utter simplicity of it all in a single moment called the here-and-now.

Buddhist doctrine includes the idea of a "middle way between extremes" -- which is used in various contexts. In general one meaning of "middle way" is, "neither extreme", or, "avoiding to the two extremes". And philosophically or logically, I see it as meaning to beware of dilemmas or false dichotomies.

I think that the question (to which that quote is a reply) is slightly tricky to answer, and wants a careful answer. It's tricky because it invites "extreme" answers, e.g. either "yes" or "no".

But the two extreme answers are possibly too extreme -- either might be untrue, or have harmful consequence.

And so the quote is trying to provide a careful answer which is neither exactly "yes" nor "no" (not careful enough perhaps, if you didn't understand the answer; but it may require a lengthy explanation).

I think there are several fundamentals of Buddhist doctrine which are important to understand. If I can invent a simile, these fundamentals are like rocks -- and the answer is like a ship, which must steer a course which takes into account and doesn't hit (doesn't contradict) any of these rocks.

• Non-self

  One of the fundamental elements of doctrine is "non-self". That means (very briefly) that there's nothing which deserves to be called "myself", furthermore that a "self-view" (e.g. "this is I, this is me, this is mine") is a dissatisfactory view and gives rise to suffering.

  Examples of self-views being an occasion for suffering include, "I am sick", "I am healthy now but I will be sick", "I want something I don't have", "I want to keep something I can't keep", "I'm better than you are", "I'm not as good as you are", and so on.

  There's a simile here (not Mahayana but perhaps all forms of Buddhism agree on this kind of topic):

  Suppose a dog on a leash was tethered to a strong post or pillar. Whether walking, standing, sitting, or lying down, it stays right beside that post or pillar.

  In the same way, an uneducated ordinary person regards form like this: ‘This is mine, I am this, this is my self.’ They regard feeling … perception … choices … consciousness like this: ‘This is mine, I am this, this is my self.’ When walking, they walk right beside the five grasping aggregates. When standing … sitting … lying down, they lie down right beside the five grasping aggregates.

  I'm not sure what solipsism is, something like "I exist and you don't", or, "I'm sure I exist and I don't know about you". And I think that those views are not Buddhist doctrine for various reasons, and one of those reasons is the "non-self" doctrine.

  I don't know if Descartes' "I think therefore I am" might be an example of solipsism. I think that according to Buddhism, proving that oneself exists is just reinforcing an unfortunate view. Possibly a more accurate formulation is, "A thought is perceived which gives rise to a (perhaps transient) sense of self".

  Also classical grammar (a.k.a. "logic") implies nouns and subjects -- "If a thought is perceived therefore a 'thinker' exists". This isn't necessarily true -- "unicorns" don't "really" exist even though there are words that describe them; and some languages (e.g. Chinese) aren't as strict as Western languages at requiring subjects and nouns in every sentence.

• Emptiness

  There's a doctrine of "emptiness" which I feel I ought to mention if only because it's one of the more specifically Mahayana doctrines.

  Correct me if I'm wrong but I think its method is to look at the components of a thing -- to observe that what one might have thought was "a thing" is just a collection of other things -- and its purpose is to see things as they really are and perhaps to not get too infatuated with or attached to things (because attachment is associated with suffering), and to "unbind".

  Like, I don't know, "I think therefore I am" is just a thought, a transient thought -- and not an especially worthy thought in my opinion, and contrary to Buddhist doctrine which recommends against self-views.

  I think the practice of emptiness applies to everything -- to things you're attracted to or may be addicted to; to things you dislike; even to mental states (not "things" at all) for example anger etc.

  So perhaps that's a reason NOT to say dogmatically "I exist" or "other people exist" -- because the non-self and emptiness doctrines are seen as beneficial, and as true or axiomatic. And so you get the circumlocution in the quote, i.e. "'we act as if' each person...".

• Nihilism

  It's explicitly wrong (a so-called wrong view) to interpret "emptiness" as "nothing".

  And what is wrong view? 'There is nothing given, nothing offered, nothing sacrificed. There is no fruit or result of good or bad actions. There is no this world, no next world, no mother, no father, no spontaneously reborn beings; no brahmans or contemplatives who, faring rightly & practicing rightly, proclaim this world & the next after having directly known & realized it for themselves.' This is wrong view...

  Instead we're taught there is, if not exactly "good and bad", and least "skilful and unskilful", "suffering and non-suffering", and "consequences of actions".

  That's another explanation or reason for, "We act as if ...".

• Sentient beings

  There's a concept of "sentient beings" in Buddhism, which might be more central to Mahayana Buddhism, which I don't know so much about. According to me that term has two meanings.

  • One is that it refers to, broadly, anyone with feelings. So includes all people, and also animals (and possibly beings of other worlds too, including gods and ghosts). A characteristic of a sentient being is that they're able to feel hurt (able to suffer), and as good Buddhists it's our duty and desire to at least avoid inflicting hurt (with what's called in the West "the Golden Rule" in its "negative or prohibitive form" i.e. "Do not treat others in ways that you would not like to be treated " being used as an argument).

    Perhaps you'd agree that the quoted statement is better than its opposite, i.e. "We DON'T act as if etc.", which might be sociopathic.

    So if you're a sentient being for example then I don't want to kill you, steal from you, make you lose your family, nor lie to you -- that's basic ethical behaviour.

  • Another interpretation, and I don't know if this is a mainstream view, is that a "sentient being" is (whether by definition or axiomatically or evidently, I don't know) "self-aware" or having a "self-view". Remembering what I wrote earlier, a self-view makes a being vulnerable to suffering: "I hurt", "I want", "I have lost", etc.

    With enlightenment perhaps "self-views" and the notion of being "a being" are discarded. See for example SN 5.10 where Vajira refuses to see herself as a being.

    Incidentally this whole topic is described as a "thicket" that's not worth thinking about.

    But that touches on your question again: perhaps a relatively "enlightened" being is not a "sentient" being in the usual sense, and being enlightened is relatively immune to any grief. EVEN SO I expect that most of us should probably act "as if" they were sentient, vulnerable.

• Compassion

  Compassion is important in Buddhism. Perhaps it's fundamental in Mahayana Buddhism.

  It seems to me that compassion is an antidote to being self-centred (where, if you remember, being "self-centred" is a cause of suffering).

  Compassion isn't "pity", note: pity implies comparison, being better-off than someone else.

A lot of what I wrote here is -- due to my unfortunate ignorance of Mahayana -- based on (possibly earlier) suttas. Maybe that's alright, because the quote says "Following the Buddha" etc. implying the earlier doctrines (the buddha-vacana, "words of the Buddha") are on-topic.

I'll conclude with a quote from the current Dalai Lama (who I suppose is as much an authority on Mahayana as anyone). There was a video where he says, I paraphrase, "If I think of myself as special -- as "His Holiness" or a "the Nobel Prize Winner" -- then I'm trapped, imprisoned [in an identity-view]. Instead I see myself as being just like everyone else."

I think that's compatible or inline with the quote you asked about, and unrelated to solipsism.

You could look into recorded sayings if you want to know how Mahayana Buddhists see the world.

There are also descriptions of celestial bodhisattvas in e.g. Santideva's Bodhicaryavatara (A Guide to the Bodhisattva's Way of Life").

If you mean specifically how do Mahayana Buddhists think about "the universe", that's more of a scholastic question, that will depends on which sutra your interlocutor bases their answer on. Tendai talks about the 3,000 worlds in a single thought, Hua-Yen says that 10,000 Buddhas can be seen preaching the dharma on the tip of one hair; both represent different interpretations of how reality is like a dream.

But no Mahayana Buddhists are not solipsists, though your quote seems more a statement of "self power" than it is 'solipsism'

Jiriki (自力, one's own strength1) is the Japanese Buddhist term for self power, the ability to achieve liberation or enlightenment (in other words, to reach nirvana) through one's own efforts. Jiriki and tariki (他力 meaning "other power", "outside help") are two terms in Japanese Buddhist schools that classify how one becomes spiritually enlightened.2 Jiriki is commonly practiced in Zen Buddhism. In Pure Land Buddhism, tariki often refers to the power of Amitābha Buddha.[3]

https://en.wikipedia.org/wiki/Jiriki_and_tariki