The paper deals with the understanding of human nature in the philosophy of T. Aquinas and D. Hume. Its aim is to highlight some of the naturalistic tendencies in the writings of both authors. Naturalism is conceived as a position which in the explanation of human nature underlines the role of natural dispositions, inclinations and capacities operating in human mind, which are out of the control of reason. Aquinas and Hume are presented as philosophers of human nature. This view is supported by their understanding of the role of natural propensities of human beings as well as the relationship between reason and emotions (passions) and also their respective explanations of moral actions.