The mind is a unit of energy consisting of 52 different forces. Amongst these forces is a force known as the evolution current (jivitindriya). It is with the force in this current that our actions get recorded.
All thoughts that appear on our mind are reflected to the evolution current and on the evolution current occurs an impression of the reflection. All such impressions remain accumulated in the mind and some of these impressions, if sufficiently well formed, can be read back by those beings who have developed minds. Our ability to remember past events of our life means that we are bringing to the surface of our mind the accumulated past impressions and reading them over.
If we are weak in mind we would not be able to remember much of our past, but that does not mean that we have destroyed those impressions or that they have disappeared from our mind. All impressions that occur on our mind remain unharmed with the evolution current and the fact that we cannot remember certain events merely means either that other subsequent impressions are covering them, or our mind is too feeble to bring them over to the surface to read them. Very often, both these causes contribute to our lack of memory.
Merely because we cannot remember the various events of our past life, we must not think that there is nothing existing with the evolution current. Every reflection that we form on our mind penetrates into the evolution current and forms in it a subtle impression which remains in permanent deposit.
When we do a good action, we form in our mind. a number of thoughts of good action, and each such thought is a reflection on our mind. And every such reflection carries into the mind an impression; these impressions accumulate with the evolution current. Similarly, when we do an evil action, we form in our mind a large number of evil thoughts in order to gain the necessary strength to do that action. These thoughts are also reflections which carry into the mind impressions and the evolution current gathers them, too, and preserves them as securely as the impressions of good action. we must remember that every impression we absorb into the evolution current of our mind is either an impression of good action (kusala kamma) or an impression of evil action (akusala kamma).
We must also remember that thoughts of action are only those which move, or tend to move, our body to action. The sensations of pleasure or pain we experience on our mind are not thoughts of action, nor does the mind develop thoughts of action during periods when we are in a state of sub consciousness. During periods when we are living in pleasure or pain, or sub consciousness, we are passing through times of reactions.
Moments of thoughts of action, therefore, are comparatively few in number, and even amongst them, only a very small proportion come within the degree of intensity to warrant our classifying them as good or evil. We need not bother about the large number of feeble thoughts occurring in our mind, because, although they too could be classed under the two categories, good and evil, the impressions they carry into the mind are very weak, and, therefore, they do not affect the evolution current very much.
The stronger thoughts of good actions and evil actions, however carry into the evolution current very deep impressions, and they remain in strength in the mind for very long periods. These impressions of good and evil actions enter the evolution current in the manner of seeds, and the evolution current receives them in the manner of a fertile seed bed. Seeds of a species produce fruits of the same species; similarly, good actions grow and bear fruit in the form of pleasure; and evil impressions grow and bear fruit in the form of pain.
Source : by Madduma Bandara (Spirit of Buddhism.com)