As we said yesterday, the central Thesis (CT) of Parmenides' argument is that "that which does not exist cannot be thought or spoken about." The argument depended on two premises:
1) A thing can be thought about only if it is possible for it to exist.
2) Anything that does not exist, cannot exist.
The contraposition of (2), led to (3) [see yesterday's post for premises 3-5]; then the hypothetical syllogism of (1) and (3) led to (4). Finally the contraposition of (4) led to (5, which is the central thesis.
The result of our analysis was that given the truth of (1) and (2), the argument did follow through to the central thesis. The question, then, is whether (1) and/or (2) is true or false. We examined (1) and found that to be most likely true. (2) however was not so lucky. There seems to have been an equivocation (change of meaning) on Parmenides' part regarding existence. It is not clear whether he meant a possible type of existence or an actual type of existence. (Clearly he would have not thought in these terms, but we can worry about his argument in this way.) If in (3) he means something like "possible existence", then the proposition becomes redundant, since the latter part of the proposition is already about possible existence. So it must be that he meant something like "Anything that does not actually exist, cannot exist." This, proposition, however, seems false, since it is true that my third child does not exist, but it is not true that it cannot exist. The counterexample shows that premise (2) is most likely false, and that the argument of Parmenides for the central thesis fails.
Since the argument that we considered depended on the correct definition of "existence", it is essential that we begin our philosophy with philosophical definitions, much like the ones that Socrates asks for in Plato's dialogues. We thus now move on to the Euthyphro
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