Final answer:
Aristotle believed that human beings have a rational soul, distinct from that of plants and animals, which endows them with the unique ability to reason and sets them apart in the natural world. This rational soul, combined with the body, constitutes the complete human being in Aristotle's philosophical framework.
Step-by-step explanation:
Aristotle's observations led him to conclude that human beings are composed of two basic parts: Soul and Body. His functional conception of the soul meant that, while all living things have souls, only humans possess a rational soul that gives them the unique capacity for reason. This rational soul sets humans apart from plants and animals due to its ability to engage in thought and decision-making. Aristotle identified rationality as the unique function of human beings, implying that human virtue is realized through the development or perfection of reason. This contrasts with other perspectives of the time, which often viewed the mind and body as separate entities.