Lacking any scientific means of pinning down the soul, the first anatomists settled on generative primacy. What shows up first in the embryo must be most important and therefore most likely to hold the soul. The trouble with this particular avenue of learning, known as , was that early first trimester human embryos were difficult to come by. Classical scholars of ensoulment, Aristotle among them, attempted to get around the problem by examining the larger, more easily obtained poultry embryo. To quote Vivian Nutton, author of , analogies drawn from the inspection of hen's eggs foundered on the subject that man was not a chicken.
The early anatomists lacked scientific methods to identify the soul's location within the body, leading them to focus on the concept of generative primacy. They believed that what developed first in an embryo was the most significant and likely to possess the soul. However, acquiring early human embryos for study was challenging, limiting their research. To circumvent this issue, classical thinkers like Aristotle turned to the more accessible embryos of poultry as a reference point in their quest to understand ensoulment.
Despite their efforts, the analogy between chicken eggs and human embryos proved inadequate. As noted by author Vivian Nutton, the fundamental difference between the two species highlighted a crucial flaw in their reasoning: humans are distinctly different from chickens. This limitation in their comparative studies underscores the difficulty of making accurate conclusions about the human soul based on the embryonic development of other animals.
