Ne optimus vel pessimus accipiatur. Die Qualitätsanforderungen an Gattungsschulden (unter Einschluss von vertretbaren und verbrauchbaren Sachen).

What constitutes fungible property is determined by the general view of the market, what constitutes generic debts is determined by the parties' agreement, and in the case of legacies by the testator's instructions. The Roman legal sources do not contain a generic term for fungible things; they speak of res quae pondere numero mensura consistunt. Consumable goods (res quae usu consumuntur) form a sub-category of fungible goods; the foodstuffs vinum oleum frumentum are mentioned. As objects of a loan, modern laws refer to 'fungible items' (apart from money); but in practice these are only consumable items. The opposite of a generic debt is a specific obligation. In German legal terminology, the distinction was made between genus debt and species debt. However, the ambiguous term species should be avoided. Species is also a subgenus. According to the prevailing view, a Roman debtor could fulfil a generic debt with the worst non-defective item. One can agree with this view if a creditor did not have the best quality guaranteed by stipulation. However, a legatee cannot, as a rule, be promised the quality of the bequest. If a slave was bequeathed in general terms (without specifying names, qualities or abilities), then it must be assumed (accipiatur) as the will of the testator that neither the legatee had the right to choose the best nor the heir to choose the worst quality. Justinian confirmed these results from three digest fountains in decrees from the years 530/531, according to which the mediocritas must be observed and a thing mediae aestimationis must be provided. According to classical sources, it was controversial whether the quality owed in the bequest of consumable goods was to be determined according to the testator's custom of consumption or according to his region. The affirmative view is preferable. According to modern codifications, a person who owes a thing defined only by class is to supply a thing of average kind and quality. Instead of this positive formulation, the negative formulation is more precise, according to which the items owed must not be below the average quality. The fulfilment of generic legacies should sometimes be determined by the needs or circumstances of the beneficiary. The object owed might then be of above-average quality. However, the legatee may not choose the most valuable item if this would harm the legitimate interests of the burdened heir.