It should also be noted that these five axioms might be combined in one,
namely, _If equals are operated on by equals in the same way, the
results are equal_. In Axiom 1 this operation is addition, in Axiom 2 it
is subtraction, and so on. Indeed, in order to reduce the number of
axioms two are already combined in Axiom 5. But there is a good reason
for not combining the first four with the fifth, and there is also a
good reason for combining two in Axiom 5. The reason is that these are
the axioms continually used in equations, and to combine them all in one
would be to encourage laxness of thought on the part of the pupil. He
would always say "by Axiom 1" whatever he did to an equation, and the
teacher would not be certain whether the pupil was thinking definitely
of dividing equals by equals, or had a hazy idea that he was
manipulating an equation in some other way that led to an answer. On the
other hand, Axiom 5 is not used as often as the preceding four, and the
interchange of integral and fractional exponents is relatively common,
so that the joining of these two axioms in one for the purpose of
reducing the total number is justifiable.
6. _If unequals are operated on by positive equals in the same way, the
results are unequal in the same order._ This includes in a single
statement the six operations mentioned in the preceding axioms; that is,
if _a_ > _b_ and if _x_ = _y_, then _a_ + _x_ > _b_ + _y_,
_a_ - _x_ > _b_ - _y_, _ax_ > _by_, etc. The reason for thus combining
six axioms in one in the case of inequalities is apparent. They are
rarely used in geometry, and if a teacher is in doubt as to the pupil's
knowledge, he can easily inquire in the few cases that arise, whereas it
would consume a great deal of time to do this for the many equations
that are met. The axiom is stated in such a way as to exclude
multiplying or dividing by negative numbers, this case not being needed.
7. _If unequals are added to unequals in the same order, the sums are
unequal in the same order; if unequals are subtracted from equals, the
remainders are unequal in the reverse order._ These are the only cases
in which unequals are necessarily combined with unequals, or operate
upon equals in geometry, and the axiom is easily explained to the class
by the use of numbers.
8. _Quantities that are equal to the same quantity or to equal
quantities are equal to each other._ In this axiom the word "quantity"
is used, in the common manner of the present time, to include number and
all geometric magnitudes (length, area, volume).
Public-domain text, read in full here on John Shaqi.
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