How can man be created from maniy (semen) which is made from blood?
Question

                How can man be created from maniy (semen) which is made from blood, which is najis (impure)?
Praise be to Allah.
      Firstly: 

    Blood is only 
    regarded as najis (impure) when it is shed and comes out. When it is still 
    inside the body and veins of the animal, it is not najis. 

    Shaykh 
    al-Islam [Ibn Taymiyah] (may Allah have mercy on him) said: 

    We do not 
    accept that blood before it emerges is najis. There should be evidence for 
    its being najis, and there can be no comparison between blood when it comes 
    out and blood that is still inside the animal, because we say that the 
    evidence for its being taahir (pure) can be demonstrated in many ways: 

    (i)

    Impurity is 
    that which is regarded as dirty and filthy. This description is not 
    attributed to these types of things except after they leave the place where 
    they are meant to be. So describing them as najis is inappropriate. 

    (ii)

    The 
    requirement with regard to anything that is najis is that it must be avoided 
    in prayer. This is not applicable to blood and other things that are inside 
    the body. 

    The blood that 
    they regard as dirty inside the body is one of the basic requirements of the 
    animal without which it cannot survive, and it may be said that the animal 
    is alive because of it. To suggest that Allah has made one of the essential 
    elements for life of both people and animals something that is impure is 
    highly unlikely. 

    Moreover, the 
    basic principle is that things are pure, and cannot be proven to be impure 
    except on the basis of evidence, and there is no evidence to prove that this 
    blood which they regard as dirty is najis. End quote. 

    Majmoo‘ 
    al-Fataawa, 
    21/598-600 

    Ibn al-Qayyim 
    (may Allah have mercy on him) said: 

    Waste matter 
    only becomes regarded as urine and stools when it departs from the place 
    where it is held, at which point it becomes regarded as impure; otherwise, 
    so long as it remains in its rightful place, it is still food and drink that 
    are clean and not dirty. Rather they become dirty after they are expelled 
    from the body. The same applies to blood: it is only impure when it has been 
    shed and has left the body. But if it is still inside the body and veins of 
    the animal, then it is not impure. End quote. 

    Badaa’i‘ 
    al-Fawaa’id, 
    3/641 

    Secondly: 

    If we assume 
    that blood is impure even when it is in its rightful place, before it leaves 
    the body of the person or animal, then maniy (semen) is still pure even if 
    it is generated from blood. That is because it is transformed, i.e., it has 
    changed from its original state of blood into something with different 
    characteristics, so it becomes pure through this transformation. What 
    matters here is not the origin from which it is transformed; rather what 
    matters is the characteristics that it has now, at the time the verdict is 
    given on it. 

    Shaykh 
    al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 

    If we assume 
    that blood is impure, then it has transformed and changed. As for their view 
    that transformation does not purify, our answer is: the one who issued this 
    long-winded fatwa has gone against scholarly consensus, because the Muslims 
    are unanimously agreed that in the case of wine, if Allah has caused it to 
    begin to change into vinegar, then it has become pure. The same applies to 
    the transformation of animals and plants. Indeed, I say: Observation shows 
    us that everything that Allah causes to change and transform from one thing 
    to another, such as turning wine into vinegar, blood into semen, the ‘alaqah 
    (blood clot) into the mudghah (lump of chewed flesh) [this is a reference to 
    stages in the development the foetus], the meat of the animal that eats 
    filth into meat that is good and pure, and the same applies to its milk or 
    eggs, and crops that are irrigated with something impure becoming pure if 
    they are irritated with pure water, and so on -- in all of these cases the 
    ruling of impurity is no longer applicable and the fact of impurity and the 
    name are no longer applicable. This is something clear and indisputable. All 
    physical creations on earth may be transformed by Allah from one thing to 
    another, and no attention is to be paid to their elements and components. 

    Majmoo‘ 
    al-Fataawa, 
    21/600-601 

    Ibn al-Qayyim 
    (may Allah have mercy on him) said: 

    When something 
    good and pure turns into something bad and impure, it is deemed to be najis, 
    such as water and food when they turn into urine and stools. So how come we 
    agree on the fact that something pure can be turned into something impure 
    and do not agree on the opposite of that? Allah brings forth good and pure 
    things from bad and impure things, and bad and impure things from good and 
    pure things. The origin does not matter; rather what matters is the 
    characteristic of the thing itself. It is not logical for the ruling of 
    impurity to remain when the name and characteristic of impurity is no longer 
    there. The ruling is connected to the name and characteristic, and changes 
    with it, according to whether it is there or not. End quote. 

    I‘laam 
    al-Muwaqqi‘een, 
    2/14 

    With regard to 
    the purity of maniy (semen), see the answer to question number
    2458.  

    And Allah knows best.