The confusion around stem cells is due, I think, in large part to a fundamental misunderstanding of the function of DNA. In the popular mind, DNA contains a complete description of a person’s body. From this perspective, the DNA in the 100 or so stem cells in a balstocyst fully describe what the person will eventually become. Now, if this were true, there would be a meaningful sense in which a blastocyst is a “potential” human being. But in fact, this is not how DNA works at all. Rather, DNA functions as a set of instructions and tools for building simple things, like proteins, for example. It’s more like a recipe book than an encyclopedia. When you understand this, it becomes easier to understand how human and other animal development occurs, starting with extremely simple structures, which over time become exponentially more complex. Leading eventually, by the way, to things like minds. This is the sense in which reductionism is unassailable.