This article focusses on a programme of research into the teaching and learning of proof inspired by Celia Hoyles. By revisiting the first of a series of projects into justifying and proving in school mathematics developed by Celia in the 1990s and by considering how the innovative research methods adopted as well as the results obtained impacted upon our own subsequent research in Brazil, we explore how her work in this area exemplifies various aspects of her contribution to the field of mathematics education as a whole. We highlight, in particular, how we have been influenced by her commitment to understand the social shaping of mathematical practices and to create mathematical cultures, tools, activities and teaching approaches that make mathematics an attractive and accessible option to the many rather than the few.