The form of reality possesses sameness and difference in all its parts. Space in its sameness is by experience found to be tri-dimensional, which means, it is determinable throughout by three coördinates; while its differences are due to the position of the points considered. For the purpose of the geometrician space is uniform, but for the purpose, say of the architect, it is not uniform. To the geometrician two congruent triangles, whether they are in the cellar or in the garret, are the same. However, to the architect the position of two congruent triangles in his design of a house is by no means the same. Every single point of space has its special and individual qualities. The whole business of science is to systematise the samenesses of experience, and to present them in such convenient formulas that they can be used for guidance in our actions. The most comprehensive formulation of the sameness of the universe as a whole has found its expression in the law of the conservation of matter and energy. This law rests upon the experience, corroborated by experiments, that causation is transformation. It states that the total amount of matter and the total amount of energy remain constant. There is no creation out of nothing and no conversion of something into nothing. EINDEUTIG BESTIMMT. After this sketch of the importance of sameness, (a subject which we have by no means exhausted,) we return to the idea of necessity. The ideas of sameness and necessity are closely related. A world of sameness is a world in which necessity rules, and necessity means regularity and order. German scientists have a very good expression to denote the formulation of events in a manner which describes them in their necessary course. If they have succeeded in finding the sameness in the instances of a certain class of events, they say that it is _eindeutig bestimmt_, which means, the sameness is determined in a way that admits of no equivocation; it is complete, representing solely and purely that feature upon the presence of which the result depends. Whatever is thus _eindeutig bestimmt_, is recognised in its necessity. The presence of that feature which makes it _eindeutig bestimmt_, determines the event to take place; and this being determined, its inevitableness, the _it will be_ of the process, is all there is to necessity. All natural phenomena that can be _eindeutig bestimmt_ are necessary in their happening. A world which with regard to the total amount of its matter and energy is the same to-day and yesterday and will be the same to-morrow, a world whose laws of form possess a sameness throughout, so that it allows of formulating and applying them in their rigidity to all facts present, past, and future, a world in which all the changes are transformations determinable with the assistance of formal laws, can be relied upon and the course of its events can be computed.