Popper and "Begging the question"

Popper says that a scientific statement is a statement that can be falsified.

"God exists"
"Schizophrenia exists"

Neither of these statements are scientific as neither can be falsified.

Depending on what is meant by the two terms.
I mean by them their conventional meanings.

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These statements - or similar ones - are also related to examples of "circular reasoning" or "begging the question" or - in Latin - petitio principii (which literally means "begging the principles" I think).

For example, in order to prove the conclusion, "Schizophrenia exists";
it may be said that "all schizophrenics have been found to have brain lesion x,y,z"; and as "schizophrenia is a brain disease"; it therefore must be true that "schizophrenia exists."

This would be an example of petitio principii because the propositions assume that schizophrenia exists, which is that which it is being attempted to prove.

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