A Pure Thought And a Pure Action
At a biological level organisms act because some internal system marks certain outcomes as beneficial and others as harmful. Modern neuroscience would describe this through reward and reinforcement systems in the brain. Whether the reward comes from outside (money, praise, food) or from inside (satisfaction, curiosity, clarity) the nervous system is still operating through the same general mechanism. Something increases the probability that the behavior will be repeated.
Strictly speaking, the claim that an action becomes “pure” when it has no reward is psychologically unrealistic. Human beings do not act without reinforcement. Even the feeling of acting morally or spiritually is itself a reward produced by the brain. However, there is still a meaningful difference between external rewards and internally generated ones, though not the mystical difference some people claim.
External rewards depend on conditions outside the organism. Money, status, approval, or results must be delivered by the environment or by other people. This makes the motivation somewhat unstable because it relies on factors the person cannot fully control. If the external reward disappears, the motivation often disappears as well.
Internal rewards arise from the activity itself whether it is understanding something difficult, solving a problem, physical movement, creating something, or even quiet concentration. The reinforcement is produced directly by the nervous system during the activity rather than after it. Because of that, these motivations tend to be more stable across changing circumstances.
This does not make them morally superior. It simply makes them structurally different in how they sustain behavior.