Rome grew, however, it became more and more likely that the owner would be away for a year or more. The Praetor extended the rules of possession to new cases, which came to form a central part of usucapio: for example, the case of the inheritor believing that formerly borrowed goods are part of his inheritance. Iusta causa and good faith Iusta causa (alternatively "iustus titulus") is a requirement, in essence, that the transfer would have been valid if not for one of the two cases mentioned above. This will be a recognised method of transfer – for example, gift or sale. This has to be in fact; it cannot rest on a mistaken belief in there being a sale or gift, which is the main difference between iusta causa and good faith in practice. Good faith is not easily defined, despite being a common concept in the Roman law. In the case of sale by a non-owner (or another defect in title), then it probably meant that the claimant believed he was becoming owner. However, the receiver of a res mancipi by traditio must surely realise the problem, but this does not prevent usucapio. The burden of proof was on anyone disputing the usucapio to show bad faith. The bad faith had to be shown at the point of the iusta causa – it was insufficient to show that the claimant later realised that the item had come from a non-owner. Stolen or taken by force The claimed must be a res habilis, an object capable of private ownership and not otherwise prohibited. Something that had at any point been stolen (furtum) or taken by force could not be usucapted. Furtum was much wider than theft in the modern criminal law (furtum was a civil action), involving most sorts of bad faith interference in another's property. This had the practical effect of extending the good faith requirement to the transferor as well as the transferee – for someone who sold, gifted or otherwise transferred the property of another in bad faith committed furtum. Indeed, that the transferor is a non-owner in fact means normally that at some point there has been furtum. Gaius, in book two of The Institutes gives two counter-examples: firstly, where a borrower has died, and his heir believes the thing to be part of his inheritance and sells it; secondly, where a man with a