Final answer:
The philosopher who questioned the existence of the Greek gods and argued for the absence of universal truth was Protagoras, known for the dictum "Man is the measure of all things." This philosophy posits that truth is relative to the individual. Figures like Socrates and Xenophanes also made significant contributions to the discussions about truth and the divine.
Step-by-step explanation:
The radical philosopher who questioned the existence of Greek gods and also posited that there is no universal truth was Protagoras. Protagoras is known for his statement "Man is the measure of all things," which signifies that truth is relative and based on individual perception. His philosophy centers on the belief that each person has their truth, contrasting with the notion of a singular, objective reality. However, the claim that there are no absolutes and that all truths are valid only relative to the individuals holding them has been a topic of debate throughout the history of philosophy. Socrates, another prominent Greek philosopher, challenged whether moral truths exist independently of the gods. Similarly, the Presocratic philosopher Xenophanes criticized the anthropomorphism of gods and proposed that divine nature is beyond human comprehension. In challenges to such relativistic views, questions have been raised about whether it is possible to reach an understanding of reality beyond the subjective relativity of individual thought. The rich philosophical discourse during the era of Greek philosophy was notably devoid of experiments and observations, relying solely on reason and logic, which dramatically shaped the development of Western thought.