_AND_ AND _OR_
When, with Boole, alternatives (A, B) are considered as mutually
exclusive, logical addition may be described as the process of taking A
_and_ B or A _or_ B. It is a great and rare convenience to have two
terms for denoting the same thing: commonly, people denote several
things by the same term, and only the Germans have the privilege of
referring to, say, _continuity_ as _Stetigkeit_ or _Kontinuierlichkeit_.
But Jevons[75] quoted Milton, Shakespeare, and Darwin to prove that
alternatives are not exclusive, and so attained first to recognized
views by arguments which were plainly irrelevant.
Of course, _and_ is often used as the sign of logical addition: thus one
may speak of one's brothers _and_ sisters, without being understood to
mean the null-class (as should be the case), or pray for one's
"relations and friends," without being sure that one's prayer would be
answered,--as it certainly would if one meant to pray for the
null-class, this being the class indicated. And a word like _while_ is
often used for a logical addition, when exclusiveness of the
alternatives is almost implied. Thus, a reviewer in _Mind_,[76] noticing
the translation of Mach's _Popular Scientific Lectures_ into American,
said of the lectures that: "Most of them will be familiar ... to
epistemologists and experimental psychologists: while the remainder,
which deal with physical questions, are well worth reading." The reader
has the impression, probably given unintentionally, that Professor
Mach's epistemological and psychological lectures are not, in the
reviewer's opinion, worth reading.
FOOTNOTES:
[75] _Pure Logic_ ..., London, 1864, pp. 76-9. Cf. Venn, _S. L._, 2nd
ed., pp. 40-8.
[76] N. S., vol. iv. p. 261.