This discussion is grounded in Aristotelian–Thomistic realism and takes the position that nursing is a practical science. As an exposition of the title statement, distinctions are made between opinion and truth, and the speculative, productive and practical sciences. Sources of opinion and truth are described and a discussion follows that truth can be achieved through knowing principles and causes of the natural kind behind phenomena. It is proposed that humans are the natural kind behind nursing phenomena. Thus, human nature provides (...) proper principles (the truth) of nursing practice. (shrink)