 Enriques argued, in his book Problemi della Scienza (1906), that this was unreasonable. The creatures would be right to ascribe a geometry to their space (and, indeed, non-Euclidean geometry) because the distorting force is beyond their control. Their geodesics are built in to the space, and it would be unreasonable of them to ascribe the paths of geodesics to the operation of a ‘force’ because that ‘force’ was not something they could even in principle manipulate. Heat, the gravitational effect of massive objects, all these distorting influences are things that can be allowed for, because they can be changed. If, in the experiment above, it were to be claimed that space is Euclidean but our candidates for straight lines are deformed it should be possible to vary the degree of deformation. One might conduct the experiment further away from any massive objects, in emptier regions of space. If different experiments gave even slightly different results one would, in accordance with Poincaré’s own criteria for changing scientific conventions, look for something in the circumstances that was responsible for the deviation of the light rays from straightness. But if no experiment were to show any difference, Enriques argued that it would be rational to conclude that light rays travelled on geodesics and the geometry of space was non-Euclidean.