B,BC."
It is a commonplace that Euclid has no right to assume, without premising
some postulate, that the two circles wzll meet in a point C. To
supply what is wanted we must invoke the Principle of Continuity (see note
thereon above, p. 235). It is sufficient for the purpose of this proposition and
of I. 22, where there is a similar tacit assumption, to use the form of postulate
suggested by Killing. " If a line [in this case e.g. the circumference ACE]
belongs entirely to a figure [in this case a plane] which is divided into two parts
[namely the part enclosed within the circumference of the circle BCD and
the part outside that circle], and if the line has at least one point common with
each part, it must also meet the boundary between the parts [i.e. the circumference
ACE must meet the circumference BCD]."
Zeno's remark that the problem is not solved unless it'is taken for granted
that two straight lines cannot have a common segment has already been
mentioned (note on Post. 2, p. 196). Thus, if A C, BC meet at F before
reaching C, and have the part FC common, the triangle obtained, namely
FAB, will not be equilateral,' but FA, FB will each be less than AB. But
Post. 2 has already laid it down that two straight lines cannot have a common
segment. .
Proclus devotes considerable space to this part of Zeno's criticism, but
satisfies himself with the bare mention of the other part, to the effect that it
is also necessary to assume that two circumferences (with different centres)
qannot have a common part. That is; for anything we know, there may be
" any number of points C common to the two circumferences ACE, BCD. It
is not until III. 10 that it is proved that two circles cannot intersect in more