 It was noted above that ad hoc hypotheses stand under suspicion for various reasons, one of which was that a hypothesis that was proposed to resolve a particular difficulty may not cohere well with the theory it purports to save or relevant background  beliefs.[9]  This could result from the fact that there is no obvious way to resolve the difficulty in a way that is wholly ‘natural’ from the standpoint of the theory itself or operative criteria of theory choice. For example, the phlogiston theory claimed that substances emitted phlogiston while burning. However, it was established that some substances actually gained weight while burning. To accommodate the latter phenomenon it was proposed that phlogiston had negative weight—but the latter hypothesis was clearly ad hoc in the sense of failing to cohere with the background belief that substances simply do not have negative weight, and with the knowledge that many objects lost weight when burned (Partington &amp; McKie 1938a: 33–38).