The Italic a is not only far more clearly distinguished from
A than is A', but it is written with one pen-stroke less,
which in the long run is a matter of importance. The
student, of course, can use A' for a whenever he finds it
convenient.
The logical investigations of Mr. A. J. Ellis, F.R.S., require
notice, because they are closely analogous to, if not nearly
identical with, my own. I am much indebted to him for
assisting me to become acquainted with his views. Not
only has he supplied me with an unpublished reprint, with
additions, of his articles in the Educational TimeSy but he
has allowed me access to the manuscripts of two elaborate
memoirs which he presented to the Royal Society, and
which are now preserved in the archives of the Society.
Some account of these investigations will be found in the
Proceedings of the Royal Society for April 1872, No. 134,
vol. XX. p. 307, and November 1873, vol. xxi. p. 497.
In the former place Mr. Ellis remarks : " The above con-
tributions are believed to be entirely original .... Jevons
first led my thoughts in this direction, but all resemblance
between us is entirely superficial." The question of resem-
blance thus raised by Mr. Ellis must be left to others to
decide 3 but in order to avoid Ipossible misapprehension,



PREFACE. xvii

I must say, that however different in symbolic expres-
siop, Mr. Ellis's logical system seems to me identical in
principle with my own. The developments of the Com-
binational Method, as described in the Educational Times
(June, July, and August, 1872), are substantially the same as
I had previously published in several papers and books.
