(7) [contranominal of (6)] A Pair of Lines, such that two
Points on one of them are non-equidistant from the other, are
non-equidistantial (i. e. are such that any two Points on either of
them, which are on the same side of the other, are non-equidistant from
the other).
*(8) A Line cannot recede from and then approach another; nor can it
approach and then recede from another while on the same side of it.
*(9) In any Circle, the inscribed equilateral Tetragon is greater than
any one of the Segments which lie outside it.
If any one of these 9 Theorems be granted as an Axiom, the rest can
be proved from it. But only 3 of them, so far as I know, have been
used as Axioms—No. 5 by Clavius, No. 8 by Dr. R. Simson, and No. 9
by myself. Clavius' Axiom requires us to assure ourselves that it
will continue true when the Lines are produced without limit;
and the strain on the imagination, caused by the effort of following
them into Infinite Space, is one to be, if possible, avoided. Dr. R.
Simson's Axiom gains a certain plausibility from the fact that, for
intersecting Lines, it admits of actual proof (see § 1.
(4)): where, however, the Lines are not known to intersect, it
is an appeal to the eye, of very doubtful force.
§ 3.
[Pg 65]
Certain universally-true Theorems, not provable from genuine Axioms,
but provable if any one of the 'Nine Quasi-Axioms,' given in § 2, be
accepted.
(1) There is a finite angular magnitude such that the angles of any
Triangle are together not-less than it.
(2) There is a Triangle whose angles are together not-less than two
right angles.
(3) There is a Triangle whose angles are together equal to two right
angles.
(4) The angles of any Triangle are together equal to two right angles.
[Euc. I. 32.]
§ 4.
Certain partially-true Theorems, not provable from any
universally-true Axioms, whether genuine or 'quasi,' but provable if
any one of themselves be accepted as an Axiom.
[N.B. By 'partially-true' is meant 'true for finite
magnitudes.' They become universally-true, if 'magnitude' be taken
to mean 'finite magnitude'; 'equal' to mean 'not differing
by a finite difference'; 'unequal' to mean 'differing by a
finite difference'; 'multiplied' to mean 'multiplied by a
finite number'; and 'intersectional' to mean 'intersectional at
a finite angle.'
These Theorems will be hereafter referred to as the 'Nine
Pseudo-Axioms,' the name being chosen to indicate that they are not
even universally true—far less self-evident.]
[Pg 66]
(1) The lesser of two homogeneous Magnitudes may be so multiplied as to
exceed the greater.
(2) A Pair of Lines, which are unequally inclined to a certain
transversal, are intersectional. [Euclid's Axiom.]
(3) [contranominal of (2)] A Pair of non-intersectional Lines
are equally inclined to any transversal. [Euc. I. 29.]
(4) A Pair of Lines, such that two Points on one of them are
non-equidistant from the other, are intersectional.
Public-domain text, read in full here on John Shaqi.
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