Though the geographical concretion may be modelled on the association, we cannot treat it perspectively, for the places being purely ideal (except the one we are at), the ideal image is not liable to deterioration by weakened perception. It may suffer degradation by forgetfulness, but that has nothing to do with perspection. _Sphericity of the Earth._ The geographical synthesis is not always formed on the pattern of a natural association. That is the first and most obvious shape to give it, and for thousands of years it appears to have answered the topographical needs of mankind. But as exploration extended it was found that the associative theory did not in some cases afford true preconception. If we travel far enough in any fixed direction we shall return to the point from which we started. This could not have been predicted from a synthesis formed on the model of a landscape. Such a return however takes place in the objects denominated spheres, and so the spherical instead of the flat form has been conceptually given to the geographical concretion. That is all that is meant by saying that the world is round. There is no world, as the mystical realist--projecting outwards his mental synthesis--imagines. There is only a scheme of spherical positions in the intellect, which facilitates the recollection of places and enables us to foresee the shortest and easiest way of reaching--_i.e._ experiencing places. The concretion is true inasmuch as the prediction is found to coincide with real experience. But that by no means implies that the places exist except when perceived in minds. XXVI--SEQUENCE Sequence is a series most resembling a procession of objects in a natural group (second category). It differs therefrom in that the objects cannot be seen together. It differs from concretion in that the order in which the objects appear cannot be altered, or if they are human and alterable we cease to treat them as a sequence. They no longer have the predictive value which moves us to form artificial groups of objects. Satisfactory examples of reasoning in sequence are less numerous than might be supposed. It is a poor category for argument. Series either occur with perfect regularity, like the seasons of the year, phases of the moon, &c., and then they rapidly become mere recollections and lose the problematical character essential to a true inference, or the connection between the objects is too casual for argumentative purposes. Darwin's theory of the formation of coral atolls is a fine argument in sequence, but the application of this theory to reefs not examined by him is hardly uncertain enough to be an argument. It is the first sequential inference that is valid--the rest are foregone conclusions.