I have a question on buddhism. Is reincarnation a Buddhist concept? If so, what is the basis?
Reincarnation is not just a Buddhist concept but a Hindu as well.
From the Karma sutra ( there is a so called fake one and a real one out there as far as i know), It stems from the questions from one of the disciples of the Buddha," Why there are differences among people? some are rich, some are poor, some endowed with wisdom , some are dumb, some having good and healthy body and some born with deform body and full of sickness " ( and why some are sinkies

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So the Buddha talks about Karma ( the concept of cause and effect) and Rebirth. Rebirth is actually link to the understanding of Karma.
The aim of buddhism is to achieve enlightment and it also means to end rebirth. Rebirth is actually suffering. to experience old age, sickness , death. No doubt there are joys in life but what buddhism teach is that such humanly joys are temporary and not permanent. Enlightenment and out of rebirth give you eternal or everlasting happiness.
Karma is the concept of cause and effect. It is not anything supernatural. For a simple understanding, if you do not study your exam ( cause), you will fail your exam ( effect) The Buddha has the understanding and wisdom of the causation of existence to the extent whom he can see why people are born in this way or that way, he can see it just like how we see a person who do not study for the exam and fail it while the hardworking student As his exam.
Let me cited an example of a child who is born in a rich family and a child who is born in a poor family in order to make this understanding easier.
You might have heard of this: " The more you give, is the more you get." "The more generous you are, the more you received" Or vice versa, The more selfish you are, the more you will be denied of opportunities or something like that.
Let's talk about the level of current birth or existence on this ( rather than rebirth, which i will touch on later).
A person who is genuine generous ( without the motive of getting anything back in return by being generous, and the generosity is done with wisdom and respect for the benefit of others and in long term) will see the world full of opportunities and abundance. He give the feeling of abundance to others by his generosity so that others have what they should have. like basic food and opportunities for progress, etc
This person who is genuine generous will want more people to have enough or more to make their life better. So what he do is to serve more. Doing business is a good way to serve more and provide more for people ( i am talking about good business people who really serve with the heart for the benefit of others and sell his products and service at the correct price rather than rip off others) business not only product the required product and service to others but in the process also make himself richer and boost the economy of the country.
In the end, his generosity ( by doing business) helps everyone to have enough in life and also help the country. and through this process, he makes his own fortune.
The above example is just to show how cause and effect works in the area generosity and there are other examples not related to setting up business but i only go for this example to foster a better understanding on how this works.
So what about a person who is not generous but selfish. he only cares about himself and in the end, no one offer opportunities to him. He only focus on himself where the world works interdependently. He put himself out of this inter-dependent system and devoid himself of opportunities.
Now to the level of rebirth to a rich family and a poor family. To explain this, you got to understand consciousness and mind perception. and also how your mind perceived is how you see the surroundings. A person who is generous will have a mind that there is always things available to give and if not, he will create it to serve it. This is the mind of abundance and the consciousness that this person has. When the person dies, this consciousness is still with him.
If he has not reached enlightenment yet, the next rebirth will follows and this generosity consciousness will go with it to the next rebirth. You can also call this the energy that will direct him to a place of abundance. ( in this case , born in a rich family) This rich family consist of family members who are also having this generosity consciousness or energy and thus it is attracted by it. ( this one you got to know molecular or cosmic energy in physics to understand more, but i am not in this area of understanding, by logic, i think it is attracted by it, if you guys think that it is a repulsion, then you go and do your research and hope you can teach me) Maybe the term, birds of the same feathers flock together holds true.
it is the same with someone who is selfish who got the selfish consciousness and reborn to a place of less opportunities and limited resources.
oh yes, i would like to add that if the person have this generosity consciousness, he will be reborn in a place of abundance, regardless he is rich or poor in his present existence.