 A second response would be to give up the tolerance principle, and adopt what Roca-Royes (2016) calls an ‘inflexible’ version of the principle that the material origin of an artefact is essential to it, holding that an artefact such as a bicycle could not have come into existence with a material composition in any way different from its actual original composition. Although this view is admittedly counterintuitive, Roca-Royes argues that it provides the best solution to the ‘Four Worlds Paradox’ to be discussed in the next section.