Students' proof abilities were explored in the context of an inquiry-based learning (IBL) approach to teaching an introductory proofs course. IBL is a teaching method that puts the responsibility for proof on students and focuses on student discussion and exploration. Data collected from each of the 70 participants included a portfolio consisting of their work on all proofs assigned during the course, as well as each student's final exam. Contrary to previous research conducted on introductory proof courses, this data analysis suggests that students developed an understanding of how to correctly use definitions and assumptions within the context of their proofs.