Supreme Court of the United States) _The Common Law_ (Boston,
Mass. 1881) is illuminating on contract as on other legal topics,
though the present writer cannot accept all the learned judge's
historical conjectures. (F. PO.)

CONTRACTILE VACUOLE, in biology, a spherical space filled with liquid,
which at intervals discharges into the medium; it is found in all
fresh-water groups of Protozoa, and some marine forms, also in the naked
aquatic reproductive cells of Algae and Fungi. It is absent in states
with a distinct cell-wall to resist excessive turgescence, such as would
lead to the rupture of a naked cell, and we conclude that its chief
function is to prevent such turgescence in unprotected naked cells. It
fulfils also respiratory and renal functions, and is comparable,
physiologically, to the contractile vesicle or bladder of Rotifers and
Turbellarians. In many species it is part of a complex of canals or
spaces in the protoplasm.

See M. Hartog, British Association _Reports_, and Degen, _Botanische
Zeitung_, vol. lxiii. Abt. 1 (1905) (see also PROTOZOA; PROTOPLASM).

CONTRADICTION, PRINCIPLE OF (_principium contradictionis_), in logic,
the term applied to the second of the three primary "laws of thought."
The oldest statement of the law is that contradictory statements cannot
both at the same time be true, _e.g._ the two propositions "A is B" and
"A is not B" are mutually exclusive. A may be B at one time, and not at
another; A may be partly B and partly not B at the same time; but it is
impossible to predicate of the same thing, at the same time, and in the
same sense, the absence and the presence of the same quality. This is
the statement of the law given by Aristotle ([Greek: to gar auto
huparchein te kai me huparchein adunaton to auto kai kata to auto],
_Metaph._ [Gamma] 3, 1005 b 19). It takes no account of the truth of
either proposition; if one is true, the other is not; one of the two
must be true.

Modern logicians, following Leibnitz and Kant, have generally adopted a
different statement, by which the law assumes an essentially different
meaning. Their formula is "A is not not-A"; in other words it is
impossible to predicate of a thing a quality which is its contradictory.
Unlike Aristotle's law this law deals with the necessary relation
between subject and predicate in a single judgment. Whereas Aristotle
states that _one or other_ of two contradictory propositions must