In an era where polarisation, alterative facts and conspiracy theories pervade public discourse, understanding differences in the way people conceive of truth and knowledge is key to unravelling contrasting worldviews. In this paper, we develop and validate the Foundations of Knowledge Questionnaire, a psychometric tool to measure trust in various ways of knowing. In Study 1, we identify six distinct factors or knowledge foundations—Expert authority, Reason, Intuition, Personal experience, Faith, and Culture. In Studies 2 and 3, we demonstrate that this factor structure provides good fit, and provide evidence for strong internal consistency, test-retest reliability, and convergent and discriminant validity for these factors. Scores on each factor also moderately predict how individuals rate the persuasiveness of arguments, but there was mixed evidence as to how well they predict general behaviour and problem solving. Most interestingly, in Study 4, these knowledge foundations (factors) correlated with belief in epistemically suspect claims, with each adding unique explanatory power. Implications, limitations, and future directions are discussed. This questionnaire offers insights into the diverse knowledge foundations underpinning human cognition and belief systems, with implications for understanding reasoning and decision-making.