There is a fundamental difference between the two comparisons.
Let us take the example given by the Buddha regarding the wheel of the wagon and the oxen which draws the wagon. There is a person who is defining the direction the wagon has to travel. The wagon, its wheels and the oxen are only objective tools who do not in anyway contribute to the direction setting. The thought process, resultant speech and the action are all stages of progression in a mental process of any individual. If the mind is corrupted then what follows in thought , speech and action are also corrupted in nature. This is the reason the Buddha gives the wheel and the Oxen example when narrating the nature of corrupt mind.
But when he talks of the peaceful and undisturbed mind and the resultant happiness, he uses the example of the ‘never departing shadow’.
Peaceful mind means, a mind which devoid of attachments. Attachments results from a dualistic nature where one stands apart from that which he is observing. This position leads to degradation in relationship as the true nature of the observed is compromised to suit that of the observer.
If one understands that the shadow has no separate existence apart from the self, he would never feel alienated and identify the shadow with any other attributes apart from one’s own. It is the light which is external to oneself which is casting the shadow and the resultant duality. Once we are endowed with this understanding, the shadow becomes an extension of the self. This knowledge is the happiness which the great Sakyamuni talks of.
The Lankavatara Sutra, which is another important sutra in the Zen tradition, says it this
Way (answers of the Buddha to Mahatmi, the bodhisattva-mahāsattva ambassador of King Ravana):
“What is meant by nonduality, Mahatmi? It means that light and shade, long and short, black and white, can only be experienced in relation to each other; light is not independent of shade, nor black of white. There are no opposites, only relationships. In the same way, nirvana and the ordinary world of suffering are not two things but related to each other. There is no nirvana except where the world of suffering is; there is no world of suffering apart from nirvana. For existence is not mutually exclusive.”The two examples given by the Buddha says volumes. The corrupted mind is the result of disjointed entities like the wheel which follows the hoof of the Ox. This is a chain of causation which at any moment can lead to separation and suffering but the shadow and the self are inseparable relationship whose duality is only due to an external projection.
A peaceful mind is one that is rooted in non-duality and which enjoys an inherent state of happiness and satisfaction.
Love to you all.