This article considers the relevance of Susan Isaacs' practice and research for twenty-first century early childhood education, reflected in two studies conducted discretely nearly a century apart that theorize young children's constructions of knowledge: Isaacs' Malting House School study and the 'Young Children Are Researchers' study. The article reviews Isaacs' work with particular focus on 'discovery, reasoning and thought', her values and three key disciplines that informed her practice and research: pedagogy, philosophy and psychology. Selected findings from Isaacs' Malting House School Study and the 'Young Children Are Researchers' study are critiqued to build the argument that not only has Isaacs' work left a powerful legacy to the field of early childhood education, but that it also has potential to benefit the field now and into the future. However, this proposition is contingent on early childhood educators who are highly knowledgeable and skilled: factors that have policy implications.