Allah SWT takes an oath by the sun and its brightness as the day is clear and bright when the sun is out and illuminating the entire earth. It is a means by which we can go about the day and enjoin good amongst the earth seeking its goodly provision and building upon beneficial knowledge that ascertains the means to be upon the way.
The moon follows after the day when its revealed easily at night time as the day unveils the different aspects of the changing environment and the night conceals the sun as it drops below the horizon and these two celestial bodies are part of the heavens that with Allahs hands he built the entire heavens above. With the term being used in the form of Allahs hands means with strength Allah SWT built the heavens above. The strength, power and will required to create the heavens is something only Allah has the power to build as the earth was spread out being made sound and well proportioned on the fitrah that the inhabitants particularly humans are born upon the innate disposition of monotheism, that is until the devils or those that seek to misguide or mislead mankind lead them away from monotheism.
This usually happens from a young age as each human being that is born upon monotheism naturally knows there is only One Creator however that changes when they are influenced and misled by others.
Allah SWT mentions that it was He who fashioned the soul and then inspired it with right and wrong, this knowledge that is built into the soul and those people are successful who purify their soul from the corruption of evil and wicked words, actions that lead to the doom of a person.
The soul is inclined towards goodness as it is fighting against the aspects of the self that tries to corrupt it. Those who manage purification of the heart through physical, emotional and spiritual correctness in the form of lifting the heart to illuminate it with sound knowledge and virtue that brings a person to fulfilling its purpose in removing all those aspects which cause harm and corrupt the individual.
If the heart is sound then the entire person is sound, this means it is required to remove anything which causes corruption or rots the individuals being from the vices of greed, envy, jealousy, pride and other negative aspects that seek to corrupt the person.
So purification is necessary in all aspects of a persons life in matters of love, family, wealth, speech, actions and from disciplining oneself they can have a sound heart that loves to seek the face of Allah the One who inspires the soul towards righteousness and gave us the knowledge to remove the harm of wrongful actions and words which corrupt the person.
Purifying the heart brings about change within the self by being inclined towards love, forgiveness and mercy and with hope the heart will make the person sound in its entire form and be complete. If the heart is sound the person is sound, otherwise corruption will destroy the person.
This is why the soul is inspired towards knowledge that it knows of right and wrong and choosing the right way, the heart knows the love of the Divine Allah SWT.