Positivism may say that there are no necessary existential truths such as ‘God exists’ or even ‘I exist’. Descartes argued his way to saying that the one thing which cannot ever be doubted was that ‘I exist’. Positivism, however, accepts as an axiom that existence cannot be a predicate (which is why questions of the form “does x exist?” are called meaningless by this school), and therefore that even statements like the conclusion of Descartes’ Cogito ergo sum are not even worthy of being called false, but are literally vacuous. To be clear, while a Cartesian could imagine a model of the world in which she did not exist, and thus could say that it is intelligible to her that she not exist, yet the verificationist will press the point that there is no verification in principle which exists to demonstrate to somebody that they do not exist. Therefore, so the Positivist says, the claim is cognitively meaningless.
Suppose we consider responding as follows: I exist. I am more sure that “I exist” is meaningful than I am that positivism is correct. So, we can just give a Moorean argument against positivism. This will or should appeal to anyone to whom Moore’s response to Scepticism is found to be appealing.
This can also be extended to Kant’s critique of existence not being a first-order predicate (that is, a predicate which enriches one’s idea of a thing). I could say that there is at least one instance where I am more sure that existence being used as a first order predicate, is meaningful, than I am that Kant is right about existence not being a first order predicate.