The literature on proof by contradiction (PBC) is nearly unanimous in claiming that this proof technique is "more difficult" for students than direct proof, and offers multiple hypotheses as to why this might be the case. To examine this claim and to evaluate some of the hypotheses, we analyzed student work on proof construction problems from homework and examinations in a university "Introduction to Proof" course taught by one of the authors. We also conducted stimulated-recall interviews with student volunteers probing their thought processes while solving these problems, and their views about PBC in general. Our results suggest that the knowledge resources students bring to bear on proof problems, and how these resources are activated, explain more of their "difficulties" than does the logical structure of the proof technique, at least for this population of students.