o cause. It is an axiom of logic that cause
is preferable to effect as a basis of those classifications designed for
scientific research. Hence the functional basis is preferred in all
cases in which it can be applied. A condenser for the fumes of zinc is
much more like a condenser for the fumes of acid or the vapor of water
than it is like the art of recovering zinc from its ores, and it employs
only one principle, to wit, heat interchange. A water-jacket for cooling
the walls of a gas-producer or glass-furnace is much more like a
water-jacket for cooling the walls of a limekiln or steam-boiler furnace
than it is like the art of gas-making or manufacture of glass articles.
In accordance with what are thought to be the correct principles,
therefore, the zinc-condenser ought not to be classified as a part of
the art of metallurgy, nor the water-jacket as a part of the art of
gas-making, merely because these instruments have a use in these arts,
but should be included, respectively, in classes based upon the more
fundamental utilities effected by them.

Although it is evident that molding a button is more like molding a
door-knob than it is like making buttons by the combined operations of
sawing, grinding, turning, and drilling, wherefore the molding of
buttons should be classified in a general plastic art rather than in a
special button-making art, yet the making of buttons by a plurality of
different kinds of operations can be placed only in a class based upon
the product, to wit, button-making. Since, therefore, the combination of
many different operations for the production of a specific article can
not be classified on the basis of any single function, it must be
classified on the basis of product. Thus by selecting essential function
as a basis when possible, and resulting effect when the functional basis
is not possible, one may approximate to the correct classification
described by Herbert Spencer as follows: "A true classification includes
in each class those objects that have more characteristics in common
with one another than any of them have with objects excluded from the
class."[7]

So it is deemed better to classify in accordance with the function or
effect it is known a means _must_ perform or accomplish than in
accordance with the _object_ with respect to which an act or acts are
directed or in accordance with some _effect_ which may or may not
result.

_Structure as a basis._--The phrase "structural