Imam has expalained the issue through his theological works as following:
Here we intend to discuss two benefits: one is the knowledge of the Day of Resurrection, and the other is the knowledge of (the necessity of) prophethood and revealed scriptures - that is, of general prophethood (al-nubuwwat al-’ammah) and systems of Divine Law (sharayi’ haqqah).
One of the issues pertaining to the soul is its state of independence (from the body), a problem which has been given more significance than any other philosophical issue by all the eminent hukama about which they have given numerous proofs and explanations.
Here we are not in a position to offer an elaborate proof of this. We shall confine ourselves to mentioning some simple preliminaries and then return to our subject.
All the physicians, scientists and anatomists unanimously agree that all the human organs, from the pia mater - which is the centre of sense perceptions and the stage for the manifestation of all psychic faculties - to the coarser parts and organs of the body, weaken, deteriorate and decline after the age of thirty or thirty-five years.
We ourselves have experienced how weakness and sluggishness overtake all the organs of the body after that age.
However, at the same time, that is at the age of thirty and forty and after that, the spiritual faculties and intellectual perceptions become more refined and gain in growth and strength, Imam further explained.
This implies that the rational faculties are not physically based, for had they been corporeal like other physical faculties, they would also have declined.
It is not right to imagine that it is the extent of intellectual activity as well as experience which strengthen the intellectual faculties, because all the physical faculties weaken and decline despite prolonged use and do not grow in strength and perfection.
This proves that the intellectual faculty is not physically rooted.
It is also improper to say that the faculty of intellect also declines with age, because, firstly, none of the physical faculties grows strong till middle age, so that it may be said that a certain organ of the body has been the centre of intellectual perception that grows in strength till middle age and then becomes weaker, thus rendering weak the faculty of intellect as well.
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