The simple negative particles are οὐ and μή. οὐ is the negative of fact and statement, and contradicts or denies; μή is the negative of the will and thought, and rejects or deprecates. The difference between the simple negatives holds true also of their compounds οὔτε μήτε, οὐδέ μηδέ, οὐδείς μηδείς, etc.
a. τὰ οὐκ ὄντα is that which does not exist independently of any opinion of the writer: τὰ οὐκ ὄντα λογοποιεῖν
b. The rarer οὐχί (οὐ-χί) denies with greater emphasis than οὐ. The form μηκέτι
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].