Karma is the law of moral causation. The theory of Karma is a fundamental doctrine in Buddhism. This belief was prevalent in India before the advent of the Buddha. Nevertheless, it was the Buddha who explained and formulated this doctrine in the complete form in which we have it today

Q: What is the cause of the inequality that exists among mankind?
Q: Why should one person be brought up in the lap of luxury, endowed with fine mental, moral and physical qualities, and another in absolute poverty, steeped in misery?
Q: Why should one person be a mental prodigy, and another an idiot?
Q: Why should one person be born with saintly characteristics and another with criminal tendencies?
Q: Why should some be linguistic, artistic, mathematically inclined, or musical from the very cradle?
Q: Why should others be congenitally blind, deaf, or deformed?
Q: Why should some be blessed, and others cursed from their births?

Either this inequality of mankind has a cause, or it is purely accidental. No sensible person would think of attributing this unevenness, this inequality, and this diversity to blind chance or pure accident.
In this world nothing happens to a person that he does not for some reason or other deserve. Usually, men of ordinary intellect cannot comprehend the actual reason or reasons. The definite invisible cause or causes of the visible effect is not necessarily confined to the present life, they may be traced to a proximate or remote past birth.
According to Buddhism, this inequality is due not only to heredity, environment, “nature and nurture”, but also to Karma. In other words, it is the result of our own past actions and our own present doings. We ourselves are responsible for our own happiness and misery. We create our own Heaven. We create our own Hell. We are the architects of our own fate.
Perplexed by the seemingly inexplicable, apparent disparity that existed among humanity, a young truth-seeker approached the Buddha and questioned him regarding this intricate problem of inequality:
“What is the cause, what is the reason, O Lord,” questioned he, “that we find amongst mankind the short-lived and long-lived, the healthy and the diseased, the ugly and beautiful, those lacking influence and the powerful, the poor and the rich, the low-born and the high-born, and the ignorant and the wise?”
The Buddha’s reply was:

Karma includes all that comes under the phrase “thought, word & deed”

“All living beings have actions (Karma) as their own, their inheritance, their congenital cause, their kinsman, their refuge. It is Karma that differentiates beings into low and high states.”
He then explained the cause of such differences in accordance with the law of cause and effect.
Certainly we are born with hereditary characteristics. At the same time we possess certain innate abilities that science cannot adequately account for. To our parents we are indebted for the gross sperm and ovum that form the nucleus of this so-called being. They remain dormant within each parent until this potential germinal compound is vitalised by the karmic energy needed for the production of the foetus. Karma is therefore the indispensable conceptive cause of this being.
The accumulated karmic tendencies, inherited in the course of previous lives, at times play a far greater role than the hereditary parental cells and genes in the formation of both physical and mental characteristics.
The Buddha, for instance, inherited, like every other person, the reproductive cells and genes from his parents. But physically, morally and intellectually there was none comparable to him in his long line of Royal ancestors. In the Buddha’s own words, he belonged not to the Royal lineage, but to that of the Aryan Buddhas. He was certainly a superman, an extraordinary creation of his own Karma.
According to the Lakkhana Sutta of Digha Nikaya, the Buddha inherited exceptional features, such as the 32 major marks, as the result of his past meritorious deeds. The ethical reason for acquiring each physical feature is clearly explained in the Sutta.
It is obvious from this unique case that karmic tendencies could not only influence our physical organism, but also nullify the potentiality of the parental cells and genes – hence the significance of the Buddha’s enigmatic statement, – “We are the heirs of our own actions.”
Dealing with this problem of variation, the Atthasalini, being a commentary on the Abhidharma, states:
“Depending on this difference in Karma appears the differences in the birth of beings, high and low, base and exalted, happy and miserable. Depending on the difference in Karma appears the difference in the individual features of beings as beautiful and ugly, high-born or low born, well-built or deformed. Depending on the difference in Karma appears the difference in worldly conditions of beings, such as gain and loss, and disgrace, blame and praise, happiness and misery.”
Thus, from a Buddhist point of view, our present mental, moral, intellectual and temperamental differences are, for the most part, due to our own actions and tendencies, both past and present.
Although Buddhism attributes this variation to Karma, as being the chief cause among a variety, it does not, however, assert that everything is due to Karma. The law of Karma, important as it is, is only one of the twenty-four conditions described in Buddhist Philosophy.
According to Buddhism, there are five orders or processes (niyama) which operate in the physical and mental realms.

They are:

Utu Niyama – physical inorganic order, e.g. seasonal phenomena of winds and rains. The unerring order of seasons, characteristic seasonal changes and events, causes of winds and rains, nature of heat, etc., all belong to this group.

Bija Niyama – order of germs and seeds (physical organic order), e.g. rice produced from rice-seed, sugary taste from sugar-cane or honey, peculiar characteristics of certain fruits, etc. The scientific theory of cells and genes and the physical similarity of twins may be ascribed to this order.
Karma Niyama – order of act and result, e.g., desirable and undesirable acts produce corresponding good and bad results. As surely as water seeks its own level so does Karma, given opportunity, produce its inevitable result, not in the form of a reward or punishment but as an innate sequence. This sequence of deed and effect is as natural and necessary as the way of the sun and the moon.
Dhamma Niyama – order of the norm, e.g., the natural phenomena occurring at the advent of a Bodhisattva in his last birth. Gravitation and other similar laws of nature. The natural reason for being good and so forth, may be included in this group.
Citta Niyama – order or mind or psychic law, e.g., processes of consciousness, arising and perishing of consciousness, constituents of consciousness, power of mind, etc., including telepathy, telaesthesia, retro-cognition, premonition, clairvoyance, clairaudience, thought-reading and such other psychic phenomena which are inexplicable to modern science.
Every mental or physical phenomenon could be explained by these all-embracing five orders or processes which are laws in themselves. Karma as such is only one of these five orders. Like all other natural laws they demand no lawgiver.
Of these five, the physical inorganic order and the order of the norm are more or less mechanistic, though they can be controlled to some extent by human ingenuity and the power of mind. For example, fire normally burns, and extreme cold freezes, but man has walked scatheless over fire and meditated naked on Himalayan snows; horticulturists have worked marvels with flowers and fruits; Yogis have performed levitation. Psychic law is equally mechanistic, but Buddhist training aims at control of mind, which is possible by right understanding and skilful volition. Karma law operates quite automatically and, when the Karma is powerful, man cannot interfere with its inexorable result though he may desire to do so; but here also right understanding and skilful volition can accomplish much and mould the future. Good Karma, persisted in, can thwart the reaping of bad Karma, or as some Western scholars prefer to say ‘action influence’, is certainly an intricate law whose working is fully comprehended only by a Buddha. The Buddhist aims at the final destruction of all Karma.

What is Karma?

The Pali term Karma literally means action or doing. Any kind of intentional action whether mental, verbal, or physical, is regarded as Karma. It covers all that is included in the phrase “thought, word and deed”. Generally speaking, all good and bad action constitutes Karma. In its ultimate sense Karma means all moral and immoral volition. Involuntary, unintentional or unconscious actions, though technically deeds, do not constitute Karma, because volition, the most important factor in determining Karma, is absent.

The Buddha says:
“I declare, O Bhikkhus, that volition is Karma. Having willed one acts by body, speech, and thought.” (Anguttara Nikaya)

Every volitional action of individuals, save those of Buddhas and Arahants, is called Karma. The exception made in their case is because they are delivered from both good and evil; they have eradicated ignorance and craving, the roots of Karma.
“Destroyed are their germinal seeds (Khina bija); selfish desires no longer grow,” states the Ratana Sutta of Sutta nipata.
This does not mean that the Buddha and Arahantas are passive. They are tirelessly active in working for the real well being and happiness of all. Their deeds ordinarily accepted as good or moral, lack creative power as regards themselves. Understanding things as they truly are, they have finally shattered their cosmic fetters – the chain of cause and effect.
Karma does not necessarily mean past actions. It embraces both past and present deeds. Hence in one sense, we are the result of what we were; we will be the result of what we are. In another sense, it should be added, we are not totally the result of what we were; we will not absolutely be the result of what we are. The present is no doubt the offspring of the past and is the present of the future, but the present is not always a true index of either the past or the future; so complex is the working of Karma.
It is this doctrine of Karma that the mother teaches her child when she says “Be good and you will be happy and we will love you; but if you are bad, you will be unhappy and we will not love you.” In short, Karma is the law of cause and effect in the ethical realm.

Karma and Vipaka

Karma is action, and Vipaka, fruit or result, is its reaction.
Just as every object is accompanied by a shadow, even so every volitional activity is inevitably accompanied by its due effect. Karma is like potential seed: Vipaka could be likened to the fruit arising from the tree – the effect or result. Anisamsa and Adinaya are the leaves, flowers and so forth that correspond to external differences such as health, sickness and poverty – these are inevitable consequences, which happen at the same time. Strictly speaking, both Karma and Vipaka pertain to the mind.
As Karma may be good or bad, so may Vipaka, – the fruit – is good or bad. As Karma is mental so Vipaka is mental (of the mind). It is experienced as happiness, bliss, unhappiness or misery, according to the nature of the Karma seed. Anisamsa are the concomitant advantages – material things such as prosperity, health and longevity. When Vipaka’s concomitant material things are disadvantageous, they are known as Adinaya, full of wretchedness, and appear as poverty, ugliness, disease, short life-span and so forth.
As we sow, we reap somewhere and sometime, in his life or in a future birth. What we reap today is what we have sown either in the present or in the past.
While we are born to a state created by ourselves, yet by our own self-directed efforts there is every possibility for us to create new, favourable environments even here and now. Not only individually, but also, collectively, we are at liberty to create fresh Karma that leads either towards our progress or downfall in this very life.
According to the Buddhist doctrine of Karma, one is not always compelled by an ‘iron necessity’, for Karma is neither fate, nor predestination imposed upon us by some mysterious unknown power to which we must helplessly submit ourselves. It is one’s own doing reacting on oneself, and so one has the possibility to divert the course of one’s Karma to some extent. How far one diverts it depends on oneself.