_Surfaces_ evidently have, moreover, the general property of exactly circumscribing volumes; and in the same way, _lines_, in their turn, circumscribe _surfaces_ and are limited by _points_. But this consideration, to which too much importance is often given, is only a secondary one.
Surfaces and lines are, then, in reality, always conceived with three dimensions; it would be, in fact, impossible to represent to one's self a surface otherwise than as an extremely thin plate, and a line otherwise than as an infinitely fine thread. It is even plain that the degree of tenuity attributed by each individual to the dimensions of which he wishes to make abstraction is not constantly identical, for it must depend on the degree of subtilty of his habitual geometrical observations. This want of uniformity has, besides, no real inconvenience, since it is sufficient, in order that the ideas of surface and of line should satisfy the essential condition of their destination, for each one to represent to himself the dimensions which are to be neglected as being smaller than all those whose magnitude his daily experience gives him occasion to appreciate.
We hence see how devoid of all meaning are the fantastic discussions of metaphysicians upon the foundations of geometry. It should also be remarked that these primordial ideas are habitually presented by geometers in an unphilosophical manner, since, for example, they explain the notions of the different sorts of extent in an order absolutely the inverse of their natural dependence, which often produces the most serious inconveniences in elementary instruction.
These preliminaries being established, we can proceed directly to the general definition of geometry, continuing to conceive this science as having for its final object the _measurement_ of extension.
It is necessary in this matter to go into a thorough explanation, founded on the distinction of the three kinds of extension, since the notion of _measurement_ is not exactly the same with reference to surfaces and volumes as to lines.
_Nature of Geometrical Measurement._ If we take the word _measurement_ in its direct and general mathematical acceptation, which signifies simply the determination of the value of the _ratios_ between any homogeneous magnitudes, we must consider, in geometry, that the _measurement_ of surfaces and of volumes, unlike that of lines, is never conceived, even in the most simple and the most favourable cases, as being effected directly. The comparison of two lines is regarded as direct; that of two surfaces or of two volumes is, on the contrary, always indirect. Thus we conceive that two lines may be superposed; but the superposition of two surfaces, or, still more so, of two volumes, is evidently impossible in most cases; and, even when it becomes rigorously practicable, such a comparison is never either convenient or exact. It is, then, very necessary to explain wherein properly consists the truly geometrical measurement of a surface or of a volume.