Abstract This paper discusses a classic Aristotelian understanding of science, nature, and methods of inquiry and proof. It then discusses nursing as a practical science and provides some demonstrations through the application of classical methods. In the Aristotelian tradition an individual substance is a unity of form and matter: form being the intelligible universal that becomes the concept, while matter is the principle of individuation. Science is mediate intellectual causal knowledge. Inquiry uses hypothetical argument, and proof that is from valid syllogistic reasoning with true premises provides necessity and certitude. This proof requires knowledge of the natural kind that gives origin to the attributes under investigation. The variability of nature is overcome by reasoning on the supposition of the end. Within the classical understanding of speculative and practical science, nursing is a practical science. As a science, nursing investigates the principles and causes of its subject, be that human beings, health or nursing behaviours as knowable nursing operations. The goal is knowledge of practice for the sake of practice, but nursing as a science is not operable. Nursing practice, that is nursing art or technologies, must complete the science. Reasoning on the supposition of the end, for example of cardiac compensation, allows one to identify necessary activities and physical states. Three causal demonstrations about decubiti show that demonstrations are possible. Classical science would teach us that the practical science of nursing studies human nature in order to identify principles and causes of health and principles and causes of nursing operations themselves. It also requires that the science ends in practical operation. Thus, proper decisions and individual actions in particular patient situations must complete nursing science.