What is karma? How are we responsible for our own karma? And can we change it?

The question of destiny or karma has greatly preoccupied philosophers in both the West and the East. One theory is that when we are born, our lives are like a sheet of white paper on which nothing is written. Each life then develops as a result of its surroundings and the forces acting on it — parents, friends, society, the dominant culture, and so on.

Buddhism, however, teaches that we have lived countless lives already. This means that we are not born as blank pages, but pages on which countless impressions have already been made. But how is this so, when we can remember nothing of our former lives, and when we so clearly die and even our bodies eventually fade away to nothing? Buddhism explains the mystery through the concept of the eternity of life. According to Buddhism, life is forever existing in the cosmos; sometimes it is manifest and sometimes latent. To understand this concept of “manifest and latent life”, consider the following. When one is deeply grieved, one feels intense sorrow, which after a while abates and one’s normal life resumes. But if one happens to recall that misfortune, the same sorrow may overwhelm one. Between the prior and present grief there is a mysterious continuity. A similar phenomenon may be observed when we sleep and then awaken; our conscious mind awakens and our body feels refreshed. But between the sleeping and awakening, our consciousness carries on.
In talking about the continuity or eternity of life, Buddhism teaches not that the body dies and life is condensed into the soul, but that the same life continues eternally, either in a manifest, seen form (life as we know it), or a latent, unseen form (death). In terms of the eternity of life, death is as much a part of living as sleep is part of the process of living. 
Karma is thus the accumulation of effects from the good and bad causes that we bring with us from our former lives, as well as from the good and bad causes we have made in this lifetime, which shapes our future.
Karma is a Sanskrit word that means “action”. Karma is created by actions — our thoughts, words and deeds — and manifests itself in our appearance, behaviour, attitudes, good and bad fortune, where we are born or live — in short, everything about us. It is all the negative and positive influences or causes that make up our complete reality in this world.
Unlike some other philosophies, though, Buddhism does not consider one’s karma or destiny to be fixed; since our minds change from moment to moment, even the habitual and destructive tendencies we all possess to varying degrees can be altered. In other words, Buddhism teaches that individuals have within themselves the potential to change their own karma.
All that we do in one lifetime affects the negative and positive balance of our karma. For example, if we are born poor in this lifetime and spend our life giving to others whatever we can give, we are making causes to change the negative karma of being poor. On the other hand, if we spend our life envying or hating rich people or even stealing from others, we are adding to our negative balance of karma. 
To understand in more depth how this works, let us go back to the moment a child is born. Buddhism teaches that when the circumstances are right for a life with a particular karma to manifest itself, this life appears, first as an embryo, then a fetus, then eventually as a newborn baby. The parents of this particular baby with this particular karma are not responsible for its karma, although they each have a karma of their own which created exactly the right situation for their child’s life to appear.
Suppose a boy is born into a family where the father is a crook and leaves the child for long periods because he is in jail. Suppose the mother breaks up the home by going off with someone else. To the child, all of this is desperately painful; he is affected by it for the rest of his life.
What is important here, however, is not so much the events that happened to the child, but the child’s reactions to the events. Such reactions — his thoughts, words and deeds — not only are his karma, they shape his future karma, too. For instance, if the child resented the father’s long absences, his way of life, he may later live a scrupulously honest life to compensate, yet his resentment may make him feel nothing is good enough. He may remain deeply ill-at-ease with himself and with others. Or, he may find it impossible to trust anyone because he was abandoned by his mother. In other words, his karma is his approach to life. Whatever this approach is will continue to direct him; thus he recreates his destiny with every thought, word and deed.
How can all this change? We can change only by changing our habitual reactions and bringing out different ones. Buddhism teaches we have all amassed negative karma throughout countless lives and that we not only experience the effects of this karma now, but we continue to recreate it.
However, the Buddhism of Nichiren Daishonin teaches that there is an area of our life that is unaffected by our karma — our Buddhahood or Buddha nature. The purpose of our Buddhist practice of gongyo and chanting Nam-myoho-renge-kyo, and regularly studying Buddhist teachings, is to reveal this area to ourselves. The purpose of our Buddhist practice for others (discussion meetings, teaching others, home visits, etc) is to help others also discover this “hidden treasure” in their lives, while allowing us to see this area working in our lives, too, so that we can come to believe in it.
Thus, by taking responsibility for our particular karma, and by chanting to reveal our Buddha nature, we can change it. As President Ikeda explained:
It (the Buddhism of Nichiren Daishonin) enables the pure life force of the Buddha state, which has existed within us since time without beginning, to well forth in unceasing currents. It changes all the tragic causes and effects that lie between and unveils the pure causes and effects which exist from the beginning less past towards the present and the future. This is liberation from the heavy shackles of destiny we have carried from the past. This is the establishment of free individuals in the truest sense of the term.