<p>The aim of this essay is to reconstruct the history of the methodological concept of adequacy, its origin, its development and shifts in conception. The concept of “adequate causation” was introduced by physiologist Johannes von Kries based on his theory of probability. He proclaimed that the contrary of an “accidental” causation is not a “necessary” causation but an “adequate” causation, which means that a result is caused with a certain, significant (objective) probability. This concept was eagerly discussed by philosophers of law and had a profound influence on Max Weber who applied it to historical and sociological analysis. Weber made “causal adequacy” a methodological postulate of sociology and coupled it with “meaning adequacy”. Alfred Schutz, who intended to provide Weber’s sociology with a philosophical foundation, rejected causal adequacy as inadequate for the social sciences and merged the two postulates into one. His definitions of the postulate of adequacy are, however, somewhat inconsistent and seem to reduce it sometimes to mere understandability, dropping the requirement of coherence of experience or, in modern terms, empirical validation. This inconsistency and its implications are discussed in some depth. Based on Schutz, Harold Garfinkel developed ethnomethodology and interpreted the postulate of adequacy in this new context. We discuss the weak and the strong version of the “unique adequacy criterion” and some of their implications. The conclusion recapitulates the crucial insights of this journey.</p>