When the parties to a contract base their consensus on a commonly held belief which is fundamental to their motivation in contracting, we say that they contract on the basis of a supposition. Should the supposition prove false, the contract will be void. While South African courts have accepted this proposition where the supposition relates to a past or present fact, the status of a supposition relating to the future has had a far more controversial history. The CPD accepted the validity of the supposition in futuro in Williams v Evans, but this decision was overruled by the SCA in recent history. This article addresses the concept of a supposition relating to a future fact, in the light of the original finding in the Williams case and the relevant case law since. The currently prevailing view is that a supposition in futuro is indistinguishable from a resolutive condition, and this article aims to reconcile this position with the finding in Williams through the medium of equating the supposition in futuro with a tacit resolutive condition. If a supposition is merely a type of tacit term, then, asks the author, why is there so much controversy about the Williams case?