It has been often thought that the opposition to reincarnation has been solely based on prejudice, when not due to a dogma which can only stand when … - William Quan Judge

Karma is an unfamiliar word for Western ears. It is the name adopted by Theosophists of the nineteenth century for one of the most important of the laws of nature. Ceaseless in its operation, it bears alike upon planets, systems of planets, races, nations, families, and individuals. It is the twin doctrine to reincarnation. So inextricably interlaced are these two laws that it is almost impossible to properly consider one apart from the other. No spot or being in the universe is exempt from the operation of Karma, but all are under its sway, punished for error by it yet beneficently led on, through discipline, rest, and reward, to the distant heights of perfection. It is a law so comprehensive in its sweep, embracing at once our physical and our moral being, that it is only by paraphrase and copious explanation one can convey its meaning in English. For that reason the Sanskrit term Karma was adopted to designate it.

2. Concentration, or Yoga, is the hindering of the modifications of the thinking principle.
In other words, the want of concentration of thought is due to the fact that the mind — here called "the thinking principle" — is subject to constant modifications by reason of its being diffused over a multiplicity of subjects. So "concentration" is equivalent to the correction of a tendency to diffuseness, and to the obtaining of what the Hindus call "one-pointedness," or the power to apply the mind, at any moment, to the consideration of a single point of thought, to the exclusion of all else... Upon this Aphorism the method of the system hinges. The reason for the absence of concentration at any time is, that the mind is modified by every subject and object that comes before it; it is, as it were, transformed into that subject or object. The mind, therefore, is not the supreme or highest power; it is only a function, an instrument with which the soul works... (Book I, Concentration)

For the effect is in the cause, and Karma produces the manifestation of it in the body, brain, and mind furnished by reincarnation. And as a cause set up by one man has a distinct relation to him as a center from which it came, so each one experiences the results of his own acts. We may sometimes seem to receive effects solely from the acts of others, but this is the result of our own acts and thoughts in this or some prior life. We perform our acts in company with others always, and the acts with their underlying thoughts have relation always to other persons and to ourselves.