Front Page Titles (by Subject) § 5.: Observations concerning Conscience - Selections from the Scottish Philosophy of Common Sense
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§ 5.: Observations concerning Conscience - Thomas Reid, Selections from the Scottish Philosophy of Common Sense 
Selections from the Scottish Philosophy of Common Sense, edited, with an introduction by G.A. Johnston (Chicago: Open Court, 1915).
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Observations concerning Conscience
I shall now conclude this essay with some observations concerning this power of the mind which we call Conscience, by which its nature may be better understood.
The first is, That, like all our other powers, it comes to maturity by insensible degrees, and may be much aided in its strength and vigour by proper culture.
A second observation is, That Conscience is peculiar to man. We see not a vestige of it in brute animals. It is one of those prerogatives by which we are raised above them.
The next observation is—That Conscience is evidently intended by nature to be the immediate guide and director of our conduct, after we arrive at the years of understanding.
It judges of every action before it is done. For we can rarely act so precipitately but we have the consciousness that what we are about to do is right, or wrong, or indifferent. Like the bodily eye, it naturally looks forward, though its attention may be turned back to the past.
To conceive, as some seem to have done, that its office is only to reflect on past actions, and to approve or disapprove, is, as if a man should conceive that the office of his eyes is only to look back upon the road he has travelled, and to see whether it be clean or dirty; a mistake which no man can make who has made the proper use of his eyes.
Conscience prescribes measures to every appetite, affection, and passion, and says to every other principle of action—So far thou mayest go, but no farther.
We may indeed transgress its dictates, but we cannot transgress them with innocence, nor even with impunity.
We condemn ourselves, or, in the language of scripture, our heart condemns us, whenever we go beyond the rules of right and wrong which conscience prescribes.
Other principles of action may have more strength, but this only has authority. Its sentence makes us guilty to ourselves, and guilty in the eyes of our Maker, whatever other principle may be set in opposition to it.
It is evident, therefore, that this principle has, from its nature, an authority to direct and determine with regard to our conduct; to judge, to acquit, or to condemn, and even to punish; an authority which belongs to no other principle of the human mind.
It is the candle of the Lord set up within us, to guide our steps. Other principles may urge and impel, but this only authorizes. Other principles ought to be controlled by this; this may be, but never ought to be controlled by any other, and never can be with innocence.
The authority of conscience over the other active principles of the mind, I do not consider as a point that requires proof by argument, but as self-evident. For it implies no more than this—That in all cases a man ought to do his duty. He only who does in all cases what he ought to do, is the perfect man.
The last observation is—That the Moral Faculty or Conscience is both an Active and an Intellectual power of the mind.
It is an active power, as every truly virtuous action must be more or less influenced by it. Other principles may concur with it, and lead the same way; but no action can be called morally good, in which a regard to what is right has not some influence. Thus, a man who has no regard to justice, may pay his just debt, from no other motive but that he may not be thrown into prison. In this action there is no virtue at all.
The moral principle, in particular cases, may be opposed by any of our animal principles. Passion or appetite may urge to what we know to be wrong. In every instance of this kind, the moral principle ought to prevail, and the more difficult its conquest is, it is the more glorious.
In some cases, a regard to what is right may be the sole motive, without the concurrence or opposition of any other principle of action; as when a judge or an arbiter determines a plea between two different persons, solely from a regard to justice.
Thus we see that conscience, as an active principle, sometime concurs with other active principles, sometimes opposes them, and sometimes is the sole principle of action.
I conclude with observing, That conscience, or the moral faculty, is likewise an intellectual power.
By it solely we have the original conceptions or ideas of right and wrong in human conduct. And of right and wrong there are not only many different degrees, by many different species. Justice and injustice, gratitude and ingratitude, benevolence and malice, prudence and folly, magnanimity and meanness, decency and indecency, are various moral forms, all comprehended under the general notion of right and wrong in conduct, all of them objects of moral approbation or disapprobation, in a greater or a less degree.
The conception of these, as moral qualities, we have by our moral faculty; and by the same faculty, when we compare them together, we perceive various moral relations among them. Thus, we perceive that justice is entitled to a small degree of praise, but injustice to a high degree of blame; and the same may be said of gratitude and its contrary. When justice and gratitude interfere, gratitude must give place to justice, and unmerited beneficence must give place to both.
Many such relations between the various moral qualities compared together, are immediately discerned by our moral faculty. A man needs only to consult his own heart to be convinced of them.
All our reasonings in morals, in natural jurisprudence, in the law of nations, as well as our reasonings about the duties of natural religion, and about the moral government of the Deity, must be grounded upon the dictates of our moral faculty, as first principles.
As this faculty, therefore, furnishes the human mind with many of its original conceptions or ideas, as well as with the first principles of many important branches of human knowledge, it may justly be accounted an intellectual as well as an active power of the mind.1
[1 ]Ibid., pp. 594-599.