This article argues that foundation, not relevance, embodies our fundamental understanding of admissible evidence. The foundation principle - only partially and somewhat obliquely stated in the Federal Rules of Evidence - is a requirement that evidence be case-specific, assertive and probably true. As such, it is a logical precondition for relevance, which is a concept subsidiary to foundation. A foundation-based theory of evidence explains, in a way that a relevance-based theory cannot, how the basic requirements of admissible evidence connect to the fundamental understanding of legal claims. Specifically, the foundation requirement is simply an extension of the general requirement that legal remedies must be based on probably true claims. A claim for relief cannot be more true than the least probable evidentiary fact needed to help prove that claim. Evidentiary facts, which must of necessity be significantly more detailed than the overall factual claim, must therefore be probably true. The foundation theory resolves certain paradoxes and longstanding problems in evidence theory, and sets out a new account of foundation, which has been widely misunderstood by evidence commentators.