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 evil God cannot hold us responsible.  Suppose again that we err, and that the evil we think we do is really good.  In this instance, first, the law of morality is violated,—­a certain, though erroneous conscience:  this is sinful.  Secondly, a bad motive vitiates an act even if the deed in itself be good.  Consequently, we incur guilt and God’s wrath by the commission of such a deed, which is materially good, but formally bad.

One may wonder and say:  “how can guilt attach to doing good?” Guilt attaches to formal evil, that is, evil that is shown to us by our conscience and committed by us as such.  The wrong comes, not from the object of our doing which is good, but from the intention which is bad.  It is true that nothing is good that is not thoroughly good, that a thing is bad only when there is something lacking in its goodness, that evil is a defect of goodness; but formal evil alone can be imputed to us and material cannot.  The one is a conscious, the other an unconscious, defect.  Here an erroneous conscience is obeyed; there the same conscience is disregarded.  And that kind of a conscience is the rule of morality; to go against it is to sin.

There are times when we have no certitude.  The conscience may have nothing to say concerning the honesty of a cause to which we are about to commit ourselves.  This state of uncertainty and perplexity is called doubt.  To doubt is to suspend judgment; a dubious conscience is one that does not function.