Final answer:
Parmenides argued the world is unchanging through logical reasoning, suggesting that existence is eternal and anything perceived as change is an illusion. This influenced Plato's theory of forms, asserting a truer unchanging reality beyond our sensory experiences.
Step-by-step explanation:
Parmenides, a Greek philosopher, posited that the world is unchanging and that all perceived change is an illusion. This idea is embedded in his doctrine which distinguishes between the Way of Truth—asserting that reality is one, unchanging, and complete—and the Way of Opinion, which holds that our sensory perceptions, which detect change, are deceptive. To justify Parmenides's claim that the world is unchanging, we must engage in a form of reasoning that eschews the senses and focuses on logical deduction.
For instance, if something exists, it could not have come from nothing, and if there ever was nothing, nothing would have continued to exist. Hence, existence implies a form of eternal being that does not change. Parmenides's influence on Western philosophy is profound, affecting Plato's theory of forms, which suggests that our changing world is only a reflection of a higher, truer reality that is eternal and unchanging. His thinking paved the way for Platonic and Aristotelian ideas that have shaped philosophical thought for millennia.