(Latin: conscientia, knowledge of one's self)
(1) The immediate intellectual perception a person has of his own existence and actions.
The more common term for this is consciousness.
(2) A judgment of the intellect, dictating what is to be done as morally right, or what is to be avoided as morally wrong, in the particular circumstances in, which one is now placed.
Conscience is an act of the virtue of prudence.
In that it is concerned with the morality of a particular case, conscience differs from a knowledge of the fundamental principles of right and wrong (synderesis), and from the understanding of their general conclusions (moral science).
Conscience is said to be certain when it dictates something as right or as wrong, without experiencing any reasonable fear of the opposite being true.
It is doubtful when it is undecided which of two contradictory views is true.
A fundamental law of ethics decrees that a person is never allowed to act with a doubtful conscience.
This means that one who seriously doubts the lawfulness of some action and nevertheless performs it, commits sin.
The same is true of one who doubts whether it is permissible to omit some action, and yet omits it.
One who is in such a state of doubt and wishes to perform (or to omit, as the case may be) the action in question should first obtain certainty of conscience.
When strict certainty cannot be obtained, it is sufficient to have moral certainty in the broad sense, viz., that which is based on arguments of probability.
All Catholic theologians admit this principle, although there is a variety of views as to how much probability in favor of liberty is required in order that a person may licitly abstain from obeying a law that probably binds him.
Conscience is true when its dictate as to what is right or wrong is correct; erroneous, when it judges what is really wrong as right, or vice versa.
The prescriptions and the prohibitions of a conscience that is invincibly erroneous must be followed.
In other words, when a person in good faith judges erroneously that he is obliged to perform or to omit a certain act, then he commits sin if he fails to perform or to omit it, as the case may be.
For the light of his own reason constitutes for every individual the ultimate subjective norm of his conduct; and God's rewards or punishments are meted out to every one, according as he has obeyed or disobeyed the voice of his own conscience.
Conscience is sometimes taken in a broader sense to signify the knowledge and the remembrance a person has of the right or wrong of his past actions.
In this sense we speak of the examination of conscience.
New Catholic Dictionary