Final answer:
While none of the provided options (Utilitarianism, Rationalism, Empiricism, Positivism) perfectly match the description of a school of thought encompassing pragmatism and pessimistic humanism with naturalistic assumptions, pragmatism itself aligns closely with this description.
Step-by-step explanation:
The school of thought that delves into pragmatism and pessimistic humanism, concerning human nature within the framework of naturalistic assumptions about existence, does not correspond neatly to the provided options of Utilitarianism, Rationalism, Empiricism, or Positivism. However, the description touches on elements often associated with pragmatism, which rejects the idea that the function of thought is simply to describe reality, and instead emphasizes its practical application and testing through human experiences. Therefore, while the question doesn't match the options exactly, pragmatism aligns with the description given, although it is not listed as one of the answer choices.
Pragmatism is associated with naturalism, a theory in realism that asserts the only relevant laws governing humanity are those of nature and the natural world. It also aligns with elements of the philosophical discussions raised by Existentialism and Post Modernism in ethics. These philosophies emphasize the role of human decision-making within a social framework, without universal or absolute principles.