1. A provision in a trust deed requiring a minimum number of trustees is a capacity-defining condition that must be fulfilled before the trust estate can be bound. When fewer trustees than specified are in office, the trust lacks capacity to contract. 2. Where a party pleads an enrichment claim specifically alleging payment made 'indebiti' in reasonable but mistaken belief that money was owing, the claim falls within the condictio indebiti rather than the condictio sine causa specialis, and the party must prove all elements of that condictio, including that the mistake was reasonable. 3. In determining whether a mistake was reasonable and excusable for purposes of the condictio indebiti, the court must consider: the relationship between parties, the conduct of the enriched party, whether that party was aware of or contributed to the mistake, the payer's state of mind, and the culpability of the payer's ignorance. 4. A creditor is not deemed to have knowledge of facts for prescription purposes under section 12(3) of the Prescription Act where the debtor has made representations warranting capacity and the debtor has acquiesced in performing the contract, even if the creditor could theoretically have discovered the incapacity by investigation.