 You find that a certain thing which has been set up as a metaphysical entity can either be assumed dogmatically to be real, and then you will have no possible argument either for its reality or against its reality; or, instead of doing that, you can construct a logical fiction having the same formal properties, or rather having formally analogous formal properties to those of the supposed metaphysical entity and itself composed of empirically given things, and the logical fiction can be substituted for your supposed metaphysical entity and will fulfill all the scientific purposes that anyone can desire. (Russell 1918, 144)