Difference  between Body and Atman
1.Atman is verily one and without parts, whereas the body  consists of many parts.
2.Atman  is the ruler of the body and is internal; the body is the ruled and is external.
3.Atman  is all consciousness and holy, the body is all flesh and impure.
4.Atman is the (supreme) Illuminator and  purity itself; the body is said to be of the nature of darkness.
5.Atman is eternal, since it is Existence  itself; the body is transient, as it is non-existence in essence.

Sankara wonders, “Yet, surprisingly the  people consider these two as one. What can be more ignorance than this?”

Confusion in  knowledge is the symptom of ignorance. It is because of ignorance that a post  is mistaken as a ghost. However the ignorance does not operate at full throttle  because there is some common link between the original and the flawed ones or  the real and the apparent like shape, color etc. between the post and the  ghost. But the confusion or the nature of ignorance is full in the case of the  body and the Atman because there is nothing common between them and on the  other hand they are opposed to each other in all respects.

The body is undergoing change every minute  and therefore that which is changing cannot be eternal. Here a problem crops  up. Although the body can be considered as non-eternal how it can be  non-existent.  The existence of the body  is there for all to see, despite its being of temporary nature. Vedanta  describes this kind of existence as vyavaharika satta, a relative  existence. On a closer examination of its real nature the so called tangible  body disappears and ceases to exist one day or the other. Hence it is referred  to as non-existent in essence, even though it may appear to exist for the time  being for those who do not care to devote more time and attention to analyze this  issue.

“The luminosity of Atman consists in the manifestation of all objects.  Its luminosity is not like that of fire or any such thing, for (in spite of the  presence of such lights) darkness prevails at night (at some place or other).”

The light of Atman makes us see everything; such light is not like that  of fire, for without fire, we cannot see anything in darkness while Atman makes  us see everything at all times. The difference between the ordinary light,  including sun light, and Atman is brought out here. Ordinary light cannot  illumine everything because in spite of such a light there are always some  places to which that light cannot reach. In the case of atman its light i.e.  Consciousness, enables us to comprehend everything at all times including the  darkness as well as light referred to above.

Sankara concludes this topic by saying “How strange is it that a person  ignorantly rests contented with the idea that he is the body, while he knows  the fact that the body is merely something belonging to him and therefore it is  apart from him. This is like a person who sees a pot knows that it merely  belongs to him and that it is apart from him”!