Abstract

The parameter-free part \(\textbf{PA}_2^*\) of \(\textbf{PA}_2\), second order Peano arithmetic, is considered. We make use of a product/iterated Sacks forcing to define an \(\omega \) -model of \(\textbf{PA}_2^*+ \textbf{CA}(\Sigma ^1_2)\), in which an example of the full Comprehension schema \(\textbf{CA}\) fails. Using Cohen’s forcing, we also define an \(\omega \) -model of \(\textbf{PA}_2^*\), in which not every set has its complement, and hence the full \(\textbf{CA}\) fails in a rather elementary way.