As the Buddha said..
“All kamma, whether good or evil, bears fruit.
There is no kamma, no matter how small, which is void of fruit.”
Essentially kamma is intention, which includes volition, will choice and decision, or the energy which leads to action. Intention is that which instigates and directs all human actions. It is the agent or prompting force in all human creation and destruction, therefore it is the actual essence of kamma, as is given in the Buddha’s words,
Cetanāhaṁ bhikkhaue kammaṁ vadāmi
Bhikkhus! Intention, I say, is kamma.
Having willed, we create kamma, through body, speech and mind. 
At this point we should broaden our understanding of this word ‘intention’ (cetanā). ‘Intention’ in the context of Dhamma has a much more subtle meaning than it has in common usage. In the English language, we tend to use the word ‘intention’ when we want to provide a link between internal thought and its resultant external actions. For example, we tend to say, ‘I didn’t intend to do it’, ‘I didn’t mean to say it’ or ‘he did it intentionally’. But according to the teachings of Dhamma, all actions and speech, all thoughts, no matter how fleeting, and the responses of the mind to the various images received through eye, ear, nose, tongue and body, and recollected in the mind itself, without exception, contain elements of intention. Intention is thus the volitional or conscious choosing of objects of awareness by the mind. It is the factor which leads the mind to turn towards, or be repelled from, various objects of awareness or mental concerns, or to proceed in any particular direction. It is the guide, the manager or the governor of how the mind responds to stimuli. It is the force which plans and organizes the movements of the mind, and ultimately it is that which determines the numerous states experienced by the mind.
One instance of intention is one instance of kamma. When there is kamma there is immediate result. Even just one little thought, although not particularly important, is nevertheless not void of consequence. It will be at the least a ‘tiny speck’ of kamma, accumulating to become an agent for conditioning the qualities of internal mental activity. If it increases, through repeated creation by the mind, or through an increase in intensity, transforming into external activity, the result becomes stronger, expanding to become character traits, physical features or results from external sources.
A destructive intention does not have to be as obvious as one of murder, for example. It may lead to the destruction of only a very small thing, such as when angrily tearing up a piece of paper. Even though that piece of paper may have no importance in itself, the action still has some effect on the quality of the mind. The effect is very different from tearing up a piece of paper with a neutral mental state, such as when throwing out a piece of scrap paper. If there is repeated implementation of such intention, the effects of accumulation will become clearer and clearer, and may extend to higher levels.
Consider the specks of dust which come floating unnoticed into a room: there isn’t one speck which is void of consequence. It is the same for the mind. But the weight of that consequence, in addition to being dependent on the amount of ‘dust’, is also related to the quality and the functioning of the mind on various levels. For instance, specks of dust which alight onto a road surface have to be of a very large quantity before the road will seem to be dirty. Specks of dust which alight onto a floor, although of a much smaller quantity, will already seem dirty. A smaller amount of dust accumulating on a table top will seem dirty enough to cause irritation. An even smaller amount alighting on a mirror will seem dirty and will interfere with its functioning. A tiny speck of dust on a spectacle lens is perceptible and can impair vision. In the same way, volition or intention, no matter how small, is not void of fruit. As the Buddha said:
“All kamma, whether good or evil, bears fruit.
There is no kamma, no matter how small, which is void of fruit.” 
In any case, the mental results of kammaniyāma are overlooked and regarded as unimportant by most people. Therefore another illustration might be helpful:
- There are many levels of clean or dirty water: the dirty water in a sewer, the water in a canel, tap water, and distilled water for mixing a hypodermic injection. Sewer water is an acceptable habitat for many kinds of water animals, but is not suitable for bathing, drinking or any more refined use. Water in a canal may be used to bathe or to wash clothes but is not drinkable. Tap water is drinkable but cannot be used for mixing a hypodermic injection. If there is no special need, then tap water is sufficient for most purpose, but if water was needed to mix with a hypodermic injection, one would be ill-advised to use tap water. This is comparable to the mind with varying levels of refinement or clarity, depending on accumulated kamma. As long as the mind is still being used on a coarse level, no problem may be apparent. But as time progresses, there may come a need to use the mind on a more refined level, at which time previous unskilful kamma will become an obstacle.
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