.'[3] The _poena conventionalis_
must not be confused with either of the titles _damnum emergens_ or
_lucrum cessans_, which we are about to discuss; it was distinguished
from the former by being based upon a presumed injury, whereas the
injury in _damnum emergens_ must be proved; and for the latter because
the damage must be presumed to have occurred after the expiration of
the loan period, whereas in _lucrum cessans_ the damage was presumed
to have occurred during the currency of the loan period. The important
thing to remember is that these titles were really distinct.[4] The
essentials of a _poena conventionalis_ were, stipulation from the
first day of the loan, presumption of damage, and attachment to a
loan which was itself gratuitous.[5] The _Summa Astesana_ clearly
maintained the distinction between the two titles of compensation,[6]
as also did the _Summa Angelica_.[7]

[Footnote 1: Ashley, _op. cit._, vol. i. pt. i. p. 399.]

[Footnote 2: Biel, _op. cit._, iv. 15, 11.]

[Footnote 3: Cleary, _op. cit._, p. 93.]

[Footnote 4: _Ibid._, p. 95.]

[Footnote 5: Cleary, _op. cit._, p. 94.]

[Footnote 6: Endemann, _Studien_, vol. i. p. 20.]

[Footnote 7: ccxl.]

The first thing to be noted on passing from the _poena conventionalis_
to interest proper is that the latter ground of compensation was
generally divided into two kinds, _damnum emergens_ and _lucrum
cessans_. The former included all cases where the lender had incurred
an actual loss by reason of his having made the loan; whereas the
latter included all cases where the lender, by parting with his money,
had lost the opportunity of making a profit. This distinction was made
at least as early as the middle of the thirteenth century, and was
always adopted by later writers.[1]

[Footnote 1: Ashley, _op. cit._, vol. i. pt. ii. p. 399.]

The title _damnum emergens_ never presented any serious difficulty.
It was recognised by Albertus Magnus,[1] and laid down so clearly by
Aquinas that it was not afterwards questioned: 'A lender may without
sin enter an agreement with the borrower for compensation for the loss
he incurs of something he ought to have, for this is not to sell
the use of money, but to avoid a loss. It may also happen that the
borrower avoids a greater loss than the lender incurs, wherefore the
borrower may repay the lender with what he has gained.'[2] The usual
example given to illustrate how _damnum emergens_ might arise, was
the case of t