Diagrams drawn on paper are copies
of the ideas in the mind, and not liable to the uncertainty that words
carry in their signification. An angle, circle, or square, drawn in
lines, lies open to the view, and cannot be mistaken: it remains
unchangeable, and may at leisure be considered and examined, and the
demonstration be revised, and all the parts of it may be gone over
more than once, without any danger of the least change in the ideas.
This cannot be thus done in moral ideas: we have no sensible marks
that resemble them, whereby we can set them down; we have nothing
but words to express them by; which, though when written they remain
the same, yet the ideas they stand for may change in the same man; and
it is very seldom that they are not different in different persons.
Secondly, Another thing that makes the greater difficulty in
ethics is, That moral ideas are commonly more complex than those of
the figures ordinarily considered in mathematics. From whence these
two inconveniences follow:- First, that their names are of more
uncertain signification, the precise collection of simple ideas they
stand for not being so easily agreed on; and so the sign that is
used for them in communication always, and in thinking often, does not
steadily carry with it the same idea. Upon which the same disorder,
confusion, and error follow, as would if a man, going to demonstrate
something of an heptagon, should, in the diagram he took to do it,
leave out one of the angles, or by oversight make the figure with
one angle more than the name ordinarily imported, or he intended it
should when at first he thought of his demonstration.
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