Catechism of the Catholic Church 1955 "The natural law is nothing other than the light of understanding placed in us by God; through it we know what we must do and what we must avoid. God has given this light or law at the creation." (St. Thomas Aquinas)

Conscience is a natural function of reason.

The human intellect is capable of the natural functioning God intended for it, including discovering the law written on the human heart, the natural moral law. This law is nothing more than the conclusions that can be drawn by reason from human nature, concerning its proper ends, and the “goods” which fulfill those ends. In knowing the ends God has determined, we also know the evils to be avoiding. 

Romans 2:14-15

When Gentiles who have not the law [of Moses] do by nature what the law requires, they are a law to themselves, even though they do not have the law. They show that what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them.

Second Vatican Council, "Gaudium et spes" 16

Deep within his conscience man discovers a law which he has not laid upon himself but which he must obey. Its voice, ever calling him to love and to do what is good and to avoid evil, sounds in his heart at the right moment.... For man has in his heart a law inscribed by God.... His conscience is man's most secret core and his sanctuary. There he is alone with God whose voice echoes in his depths.

Conscience is not a private revelation by God to us.

Conscience is a judgment of the intellect about what we must do, and what we must avoid.

Catechism of the Catholic Church 1778

Conscience is a judgment of reason whereby the human person recognizes the moral quality of a concrete act that he is going to perform, is in the process of performing, or has already completed.

Second Vatican Council, Declaration "Dignitatis Humanae" 3

... In all his activity a man is bound to follow his conscience in order that he may come to God, the end and purpose of life. It follows that he is not to be forced to act in manner contrary to his conscience. Nor, on the other hand, is he to be restrained from acting in accordance with his conscience, especially in matters religious.

Pope John Paul II, Encyclical "Veritatis Splendor" 58

Conscience is like God's herald and messenger; it does not command things on its own authority, but commands them as coming from God's authority, like a herald when he proclaims the edict of the king. This is why conscience has binding force.

Conscience must be properly formed to know and judge rightly

While everyone has the faculty of reason, not everyone actually comes to know the moral law. For this reason God revealed the precepts of the law through Moses and the law of charity through Christ. Divine revelation makes moral truth accessible to all people in every time and place.

Pope Pius XII, Encyclical "Humani generis"

However, the precepts of natural law are not perceived by everyone clearly and immediately. In the present situation sinful man needs grace and revelation so moral religious truths may be known "by everyone with facility, with firm certainty and with no admixture of error."

Second Vatican Council, Declaration "Dignitatis Humanae" 14

In the formation of their consciences, the Christian faithful ought carefully to attend to the sacred and certain doctrine of the Church. For the Church is, by the will of Christ, the teacher of the truth. It is her duty to give utterance to, and authoritatively to teach, that truth which is Christ Himself, and also to declare and confirm by her authority those principles of the moral order which have their origins in human nature itself.

The conscience of the individual is not autonomous.

Although the judgment of conscience must be obeyed, it is not sovereign in the sense that it determines what is good and evil.

St. John Henry Newman, "Letter to the Duke of Norfolk"

When men advocate the rights of conscience, they in no sense mean the rights of the Creator, nor the duty to Him, in thought and deed, of the creature; but the right of thinking, speaking, writing, and acting, according to their judgment or their humour, without any thought of God at all. ... Hence conscience cannot come into direct collision with the Church's or the Pope's infallibility, which is engaged in general propositions, and in the condemnation of particular and given errors." (Letter to the Duke of Norfolk)

Pope St. John Paul II, Encyclical "Dominum et Vivificantem" 36

For God the Creator is the one definitive source of the moral order in the world created by him. Man cannot decide by himself what is good and what is evil—cannot "know good and evil, like God". ... To man, created to the image of God, the Holy Spirit gives the gift of conscience, so that in this conscience the image may faithfully reflect its model, which is both Wisdom and eternal Law, the source of the moral order in man and in the world.

Pope St. John Paul II, Encyclical "Veritatis Splendor" 32

The individual conscience is accorded the status of a supreme tribunal of moral judgment which hands down categorical and infallible decisions about good and evil. To the affirmation that one has a duty to follow one's conscience is unduly added the affirmation that one's moral judgment is true merely by the fact that it has its origin in the conscience. ... Once the idea of a universal truth about the good, knowable by human reason, is lost, inevitably the notion of conscience also changes.

Catechism of the Catholic Church 1783

Conscience must be informed and moral judgment enlightened. A well-formed conscience is upright and truthful. It formulates its judgments according to reason, in conformity with the true good willed by the wisdom of the Creator. The education of conscience is indispensable for human beings who are subjected to negative influences and tempted by sin to prefer their own judgment and to reject authoritative teachings.

Conscience is subject to Church teaching in matters of both faith and morals

Vatican I, Dogmatic Constitution "On the Catholic Faith" Ch. 3, §8

Wherefore, by Divine and Catholic faith all those things are to be believed which are contained in the word of God as found in Scripture and tradition, and which are proposed by the Church as matters to be believed as divinely revealed, whether by her solemn judgment or in her ordinary and universal magisterium.

Catechism of the Catholic Church 1785

In the formation of conscience the Word of God is the light for our path, we must assimilate it in faith and prayer and put it into practice. We must also examine our conscience before the Lord's Cross. We are assisted by the gifts of the Holy Spirit, aided by the witness or advice of others and guided by the authoritative teaching of the Church.

Catechism of the Catholic Church 2035

The supreme degree of participation in the authority of Christ is ensured by the charism of infallibility. This infallibility extends as far as does the deposit of divine Revelation; it also extends to all those elements of doctrine, including morals, without which the saving truths of the faith cannot be preserved, explained, or observed.

Catechism of the Catholic Church 2036

The authority of the Magisterium extends also to the specific precepts of the natural law, because their observance, demanded by the Creator, is necessary for salvation.

The individual is obliged to form a Right Conscience with respect to the truth.

An objectively correct conscience with respect to the content of the moral law is a right conscience. Since different areas of morality can have different moral principles (e.g. social teaching versus sexual), one can have a right conscience on one matter and not on another.

Our obligation is to use due diligence (reasonable care) in acquiring the knowledge needed for forming a right judgment for each particular moral decision. One may remain invincible ignorant of the truth despite due diligence, but if there is a lack of due diligence there is moral fault.


Vatican II, Pastoral Constitution "Gaudium et spes" 16

... [T]he more right conscience holds sway, the more persons and groups turn aside from blind choice and strive to be guided by the objective norms of morality. Conscience frequently errs from invincible ignorance without losing its dignity. The same cannot be said for a man who cares but little for truth and goodness, or for a conscience which by degrees grows practically sightless as a result of habitual sin.

Catechism of the Catholic Church 1791

This ignorance can often be imputed to personal responsibility. This is the case when a man "takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded through the habit of committing sin." In such cases, the person is culpable for the evil he commits.

Catechism of the Catholic Church 1792

Ignorance of Christ and his Gospel, bad example given by others, enslavement to one's passions, assertion of a mistaken notion of autonomy of conscience, rejection of the Church's authority and her teaching, lack of conversion and of charity: these can be at the source of errors of judgment in moral conduct.

The obligation to seek a Certain Conscience.

A conscience which, after due diligence, is certain that its judgment is the correct one is a certain conscience. The certainty required is a not an absolute certainty, but a moral certainty—that which a reasonable person would judge to be correct, without a prudent fear of being wrong.

Moral certainty is the standard required in both morality and law. We should not be afraid to act on a moral certainty; we do it all the time! For example, we drive to work without an absolute certainty we won’t kill, or be killed, along the way. Rather, we have a moral certainty that IF we drive lawfully, and show reasonable care, we and those with whom we share the road will arrive safely at our destinations.