Final answer:
Parmenides believed that non-being could not be thought because he viewed reality as permanently changeless and all change as an illusion.
Step-by-step explanation:
Parmenides, a pre-Socratic philosopher, believed that the concept of non-being could not be thought because 'being' is all that exists in a permanent, changeless state. According to Parmenides, all change is an illusion, and the reality that we perceive through our senses is deceptive. His philosophy emphasizes the need to trust reason over the senses, leading to the assertion that reality is being, not becoming, implying that change (non-being) is not a true reality.