I don't understand why it has to be an adult soul instead of, for example, a baby one. Isn't it the same thing, considering it wouldn't have any memory of a previous life?
maybe because, according to Avicenna's logic, babies' soul is not a fully form soul. Avicenna's soul come about from an arrangement of matter, these connected bunch of matter or body, stores in it an 'agent of intellect' that the soul accessed whenever it senses the environment, and both the senses and these 'agent of intellect' sum inside the soul into an parts characteristic of 'knowledge' into the soul and created who the soul think he/she were. So, according to this logic, the babies' soul is definitely not yet reaching its final form and thus it's not same as adult. You can try remember the thought about "reincarnation", a process which is impossible according to Avicenna's soul, because a body automatically carried its own portion of soul and you will have trouble transplanting an foreign 'active intellect' into this body lest you will have 2 soul in it.
Coz the soul "grows" and "matured" by knowing. That's why the more we know the reality of things we become wiser. This growth and maturity is not defined by time tho, it's rather defined by knowing
Avicenna was fond of wine, and, on being reproached for his defiance of the Koran, replied: “Wine is forbidden because it excites quarrels and bad passions, but I, being preserved from excesses by my philosophy, drink wine to sharpen my wits.”
