The medieval Catholic philosopher and theologian, [[Duns Scotus]], famously argued for this doctrine as follows: ''Potuit, decuit, ergo fecit'', which is [[Latin]] for ''God could do it, it was appropriate that God do it, therefore God did it''. Scotus argued that since God could have caused Mary to be miraculously conceived free of original stain, and it was fitting for God to do so, therefore God must have so done.
[[Protestant]] positions on this topic differ. They believe Mary to be a normal human being, a sinner, who was used by God to bring Christ into the world. This is a reasonable point of view given the lack of Biblical support for Mary being "exempt from all stain of original sin." The primary Biblical evidence cited by Catholics is the passaage passage in which the angel announces to Mary that she would be the mother of the Savior and uses these words, "Hail, Mary, full of grace...."
Does "full" mean sinless, and if so, does it mean that she always was so? Unless the answer to the latter question is "yes," the doctrine of the Immaculate Conception is not affirmed. Hebrews 10:5 says, "a body thou hast prepared for me", which has been applied to the immaculate conception of the body of Mary in advance preparation for the incarnation of Jesus, so that at his incarnation he would assume his sinless flesh from her body. The same passage has also been limited as applying instead only to the moment of Jesus' incarnation in her womb, so that the flesh of his body alone was preserved free of sin.