Per the "American Heritage Dictionary of Science," there are four defining characteristics of life: "growth, reproduction, metabolism, and response to stimuli." Numerous scientific publications show that these elements of life all occur during or within moments after fertilization. Thus, clinical literature is explicit that each new human life begins at fertilization. For example, the medical textbook "The Developing Human: Clinically Oriented Embryology" states that fertilization "is the beginning of a new human being." Likewise, "The Encyclopedia & Dictionary of Medicine, Nursing & Allied Health" states that fertilization creates "a new life." Detailed documentation of these facts and others is provided at the links below.