Immediate animation: Thomistic principles applied to Norman Ford's objections.
Norman Ford and other scholars argue that the human embryo cannot have a human soul until after week two of fertilization. They claim that the individuality proper to ensoulment requires the development of the primitive streak and excludes the possibility of twinning. Their arguments are refuted by principles of philosophical realism inspired by Aristotle and Aquinas. According to these principles, the new substance formed by the union of the paternal and maternal DNA has a rational soul. The zygote, to be human, requires a human soul that determines its human development. The primitive streak is part of this development, not a condition of it. Twinning does not disprove the existence of a previous embryo with a human soul. Instead it indicates the existence of a new embryo derived from some totipotent cells that separate from the first embryo.