One of the most critical issues in determining attitudes toward embryo and stem cell research is defining the point at which life starts. Does it begin at the moment of fertilization, at the time of "ensoulment," or only at the point in development when the fetus is viable outside the uterus? Those opposed to human stem cell research believe that human life and personhood start at the moment of the fertilization of the ovum, i.e. at the zygote stage; whereas those favoring such research hold that human personhood does not start until later in the embryo's development, and, therefore, that benefits to the community outweigh concerns for the "early" embryo. These differing attitudes arise from fundamental religious beliefs and varying interpretations of philosophical principles.
Historically, there has been a progressive evolution in Roman Catholic thought on this matter. In the early days of the Church, leaders like St. Augustine and St. Thomas Aquinas followed Aristotle’s view, suggested by Talmudic scholars before him that human life starts with ensoulment at about forty days of gestation. Popes Innocent III in 1211 A.D. and Gregory XIII in 1550 accepted this thinking. In the eighteenth century, the influence of what was perceived as social laxity, especially with respect to an increased use of abortion, began to change the Church’s thinking. A major impetus for change came with the discovery in 1827 of ova in the female body and in 1875 of the process of their fertilization by sperm, leading Pope Plus IX to declare that ensoulment and human life started at fertilization. His successors have reaffirmed this view, which is also firmly held by leaders of the Greek Orthodox Church and by several other Christian denominations........
