The idea of "powerful knowledge" as a curriculum principle has led to extensive discussion. It has been framed as a way of bringing knowledge back into curriculum thinking in the light if its absence in curriculum theory. However, questions have been raised regarding powerful knowledge as a knowledge-based curriculum principle; questions about difficulties in converting it into curriculum content, as well as putting knowledge-as-an-end-in-itself above educational aims. The focus of this article is on how powerful knowledge can be conceived of as capacity-building curriculum content, as well as how this is related to the epistemological underpinning of the idea of powerful knowledge. Knowing as the capacity-building aspect of powerful knowledge is highlighted as is a widening of the concept of knowledge to also include tacit aspects in what counts as knowledge. Through revisiting Paul Hirst's forms of knowledge as well as his practice turn, I argue that Hirst became stuck in a tension similar to Michael Young and Johan Muller. By a shift of focus from powerful knowns to powerful knowings the tension between a knowledge and practice-based view of the curriculum can be dissolved.