« ΠροηγούμενηΣυνέχεια »
propositions as are only known to be true in the instances in which
experiment has actually been made. For example, a cooper knows that in every instance where he has
tried it, the distance that went exactly round the rim of his cask at six times, was the distance to be taken in his compasses in order to describe the head that would fit. But he does not know the reasons why this will necessarily be the case, not only in the instances which he has tried, but in all which he has not tried also. And to supply these reasons, is the object of Euclid's 15th Proposition of the Fourth Book.
XXVIII. The conduct of a regularly-ordered demonstration in
geometry, divides itself into five parts, which succeed one another, and are named, as follows:
The first conveys a statement of the universal proposition to be finally established. Which is called the enunciation.
The second presents to the eye, or to the imagination, a particular instance, in respect of which the truth of the proposition is to be established; with an understanding always, that nothing shall be done in respect of this particular instance, which would not be equally applicable to any other particular instance that could be substituted. Which is called the specification.
The third performs, or supposes to be performed, such operations [as, for example, drawing or dividing lines, describing figures, fc.] as are to be made use of in the further progress of the demonstration. Which is called the construction. Sometimes the construction, or part of it, precedes the specification. And sometimes no construction is required.
The fourth derives from all that has preceded, the establishment of the proposition in the instance presented in the specification. Which is called the proof.
The fifth extends the conclusion to all instances that come under the terms conveyed in the enunciation. Which is called the generalization.
XXIX. When all the instances to which a proposition may be
applied, cannot be included under one specification or one construction, the proposition is said to divide itself into two or more Cases ; which may in fact be considered as so many distinct propositions, each of which has, or is capable of having,
its separate enunciation, specification, construction, &c., the which, taken together, amount to the establishment of the
universal proposition. XXX. What is called the converse of a proposition is, when the
premises and the conclusion are made to change places, and the proposition so arising is presented as a new proposition. For example, if the original proposition is, that magnitudes which are
equal to the same, are equal to one another; the converse of this proposition is, that magnitudes which are equal to one another, are
equal to the same. XXXI. What is called the negative of a proposition is, when
a negation is inserted both in the premises and the conclusion, and the proposition so arising is presented as a new proposition. For example, if the original proposition is, that if of equals one be
greater than some thing else, the rest are severally greater than the same; the negative of this proposition is, that if of equals one be not greater than some thing else, the rest are severally not greater
than the same. XXXII. What is called the contrary of a proposition is, when
both the premises and the conclusion are altered, not merely by the insertion of a negation, but by being changed into something of a positively contrary kind. For example, if the original proposition is as in the last article ; the
contrary of this proposition is, that if of equals one be less than
some thing else, the rest are severally less than the same. SCHOLIUM. Neither the converse, the negative, nor the contrary of any proposition, is to be admitted to be true, till it has been demonstrated as a distinct proposition. For till this be done, it is impossible to know whether it is true or not. For example, it is shown in the sequel, that if one angle of a triangle is greater than a right angle, the other two are necessarily less than right angles. The converse of which is, that if two angles of a triangle are less than right angles, the other is necessarily greater than a right angle. The negative is, that if one angle of a triangle is not greater than a right angle, the other two are not less than right angles. The contrary is, that if one angle of a triangle is less than a right angle, the other two are greater than right angles. Every one of which is totally and absolutely false.
(The Nomenclature of the First Book is continued without inter
ruption of numbers in page 50, after the Intercalary Book.)
to one another.
B * I. Nomen
For because A is equal* to C, if their boundclature 14.
aries were applied to one another A and C would coincide ; or else might be made capable of doing so, by a different arrangement of parts. And because B is equal to C, in like manner would B and C. But because A and B would each coincide with C; if the boundaries of both could be applied to those of C at
once, A and B would coincide with one another; wherefore A and +1.Nom. 14. B are equal t. And in the same manner if the magnitudes
equal to C were more than two.
And by parity of reasoning, the like may be proved in every other instance. Wherefore, universally, magnitudes which are equal to the same, are equal to one another. Which was to be demonstrated. COROLLARY 1. If of equals, one be equal to some thing else, the rest are severally equal to the same. Let A and B be equal, and let B be equal to C. A shall also be
equal to C.
A and C are equal.
else; the rest are severally greater, or less, than the same.
B 11. Nom. 15. For since B is greater1 than C, a certain mag
nitude may be taken from B, and the remainder be equal to C; or B is equal to the sum of the magnitude equal to C, and of a certain magnitude besides. But because A and B are equal, A (by Cor. 1) is also equal to the sum of the magnitude equal to C, and of the certain
magnitude besides. Therefore a certain magnitude may be taken * I. Nom. 15. from A, and the remainder be equal to C; or A is greater* than C.
And in a similar manner if one were less.
than C; therefore (as has been proved above) A also is greater than C; that is, C is less than A. And in a similar manner if C were
greater than B.
A is equal to D. But C is also equal to D ; therefore (by Prop. I.)
A and C are equal.
other, is added another magnitude C. The sum of A and C is
equal to the sum of B and C.
boundaries being applied to one another they A
if its boundaries were applied to those of the magnitude which is the *I. Nom.14. sum of B and C, would coincide with it, and therefore is equal* to it.
Second Case; If either C be added in some other manner than as above;
or if A and B would not coincide with one another, but are only such as by a different arrangement of parts might be made to do so ; the magnitudes which are the sums, may be made capable of coin
ciding by merely altering the arrangement of parts, and therefore +1.Nom. 14. they are equal +.
COR. 5. If equals be added to equals, the sums are equal.
Let A be equal to B, and C to D. The sum of
be added to these.
and a certain magnitude besides. But if to the equals were added
one is added a certain magnitude besides, that sum is made greater. So also [with slight verbal alterations) if unequals be added to the same.
Cor. 7. If equals be taken from equals, the remainders are equal.
For, if this be disputed, let it be assumed that one remainder is greater
than the other. Add each to the equals that were before taken away; and because to equals unequals are added, that sum in which the unequal was greatest, (by Cor. 6) must be greater than the
other. Which is impossible; for the things by the hypothesis were •I. Nom.26.
equal to begin with. The assumption", therefore, which involves this impossible consequence, cannot be true ; or one remainder cannot be greater than the other. And because one is not greater
than the other, they are equal. So also [with slight verbal alterations] if equals be taken from the same,
or the sume from equals.
unequal. And that remainder is greatest, in which the unequal
besides. But if the equals were taken from equals, the remainders
sides, that remainder is made greater. So also [with slight verbal alterations] if from unequals be taken the
CoR. 9. If unequals be taken from equals, the remainders are
unequal. And that remainder is least, in which the unequal
them were taken from the equals, the remainders (by Cor. 7) would
besides, that remainder is made less. So also [with slight verbal alterations] if unequals be taken from the
CoR. 10. Magnitudes which are double of the same or of equal
magnitudes, are equal to one another. And so if, instead of
B For, to take the double of A, is to add to it a magnitude C that is equal to A ; and to take the double of B, is to add to it a magnitude D that is equal to B. But because A and B are equal to one another, (by Cor. 3) C and D are equal to one another. Therefore (by Cor. 5) the sum of A and C is equal to the sum of B and D,
And in like manner if to the sum of A and C be added another magnitude equal to A, and to the sum of B and D another magnitude equal to B. And so on.