f adultery (to which Isidore in his _Book of Etymologies_
gives the fanciful derivation of "_ad alterius thorum_") was not by the
canon law as received in England restricted by the operation of
arbitrary rules. It was never, for example, required, as by the law of
Mahomet, that the act should have been actually seen by competent
witnesses, nor even that the case should be based on any particular kind
of proof. It was recognized that the nature of the offence almost
inevitably precluded direct evidence. One rule, however, appears to have
commended itself to the framers of the canon law as too general in its
application not to be regarded as a principle. The mere confession of
the parties was not regarded as a safe ground of conviction; and this
rule was formulated by a decretal epistle of Pope Celestine III., and,
following it, by the 105th of the Canons of 1604. This rule has now been
abrogated; and no doubt it is wiser not to fetter the discretion of the
tribunal charged with the responsibility of deciding particular cases,
but experience of divorce proceedings tends to confirm the belief that
this rule of the canon law was founded on an accurate appreciation of
human nature.

Although, therefore, with the above exception, no strict rules of the
evidence necessary to establish adultery have ever been established in
the English courts, experience has indicated, and in former days judges
of the ecclesiastical courts often expressed, the lines upon which such
proof may be expected to proceed. It is necessary and sufficient, in
general, to prove two things--first the guilty affection towards each
other of the persons accused, and, secondly, an opportunity or
opportunities of which, if so minded, their passion may have been
gratified. It is obvious that any strong proof on either of these points
renders strict proof on the other less needful; but when proof on both
is afforded, the common sense of a tribunal, acting with a knowledge of
human nature, may be trusted to draw the inevitable conclusion.

The definition of cruelty accepted by the ecclesiastical courts as that
of the canon law is the same as that which prevails at the present time;
and the view of the law taken by the House of Lords in _Russell_ v.
_Russell_ (1897 App. Cas. 395) was expressly based on the view of
cruelty taken by the authorities of the ecclesiastical law. The best
definition by older English writers is probably to be found in Clarke's
_Praxis_ (p