When a person commits a crime...
When a person commits a crime, especially a heinous one such as masterminding mass killings, as the Nazis did in WWII, he is expected to be brought to justice, even if he committed the crimes half a century ago. Science tells us that the average person's body cells are nearly all renewed every six years. For all practical purposes we are not physically the same person we were, say ten years ago.
Why then should the Nazi officer, whose body bears little resemblance to the one which carried out the atrocities, be apprehended, tried, and punished? We intuitively understand that we are not losing some parts of our being which must be replaced. The unity of identity which we feel is not justified only through physical continuity.
So there seems to be universal recognition of the fact that, what animates a person is ultimately constant, and is held responsible for the whole of the person body and spirit. Because the two are so closely interwoven, and because we humans are so deeply anchored in this physical, material world, we tend to identify with the body rather than the spirit.
In Islam there is no doubt that the spirit is the essence of the person, and the body a vehicle for the manifestation of the soul in this world, and a means for its works. There is a set of reasons proving the existence of the spirit through the study of our knowledge. If we prove that our knowledge is not material, it will be clear that we are not merely bodies. For example, it is obvious that a greater thing cannot be placed in a smaller thing.
A big box can easily accommodate an smaller one put into it. The reverse does not hold. Then imagine gazing at a beautiful forest for a while, savouring all that stands before you. Later on when you remember the experience with all its grandeur, little thought is given to the size of the forest, which is, in a way, now stored within your memory. Whatever one might call it the idea, picture, or the experience of the forest fits neatly within a person's mind which is rather limited in size.
We wonder how a forest, with all its characteristics and size can fit into our consciousness. The problem is not solved if we imagine small microfilms, for instance, which are pictures of what we see, and are in one way or another, stored in our minds. Even extremely small microfilms have dimensions, which, when added together, would soon leave no more room in our minds for anything else.