witness present at every deed, good or evil, of our lives.

Conscience should always tell the truth, and tell it with certainty.
Practically, this is not always the case. We are sometimes certain that
a thing is right when it is really wrong. There are therefore two kinds
of conscience: a true and a certain conscience, and they are far from
being one and the same thing. A true conscience speaks the truth, that
is, tells us what is truly right and truly wrong. It is a genuine echo
of the voice of God. A certain conscience, whether it speaks the truth
or not, speaks with assurance, without a suspicion of error, and its
voice carries conviction. When we act in accordance with the first, we
are right; we may know it, doubt it or think it probable, but we are
right in fact. When we obey the latter, we know, we are sure that we
are right, but it is possible that we be in error. A true conscience,
therefore, may be certain or uncertain; a certain conscience may be
true or erroneous.

A true conscience is not the rule of morality. It must be certain. It
is not necessary that it be true, although this is always to be
desired, and in the normal state of things should be the case. But true
or false, it must be certain. The reason is obvious. God judges us
according as we do good or evil. Our merit or demerit is dependent upon
our responsibility. We are responsible only for the good or evil we
know we do. Knowledge and certainty come from a certain conscience, and
yet not from a true conscience which may be doubtful.

Now, suppose we are in error, and think we are doing something good,
whereas it is in reality evil. We perceive no malice in the deed, and,
in performing it, there is consequently no malice in us, we do not sin.
The act is said to be materially evil, but formally good; and for such