An experimentally useful criterion is that heritable information exists for any attribute of a living organism in which a change can be inherited in the absence of its cause. Structural changes in germline DNA can be passed to future generations and thus fulfil the criterion: most externally caused phenotypic changes clearly do not. The qualification in the criterion concerning the absence of cause excludes changes confounded with environmental correlation between relatives and is of crucial importance in experimental attempts to demonstrate the inheritance of acquired characters. The criterion has a defect in that it excludes certain attributes against common sense, e.g. DNA sequences in which only dominant lethal changes can occur. An appeal to analogy might however be made in such situations.