Summa summae S. Thomae, Tomus II

I say 1° : Conscience, quasi cum alio scientia [knowledge-with-another], or the conjunction and application of knowledge to some particular fact ¹ , can be defined: « The judgment or dictate of the practical intellect dictating from common principles that something in particular here and now is to be shunned as evil, or done as good; » or, as commonly said, « dictating concerning the goodness or malice of those things which are to be done by us here and now. » Conscience therefore is the conclusion of the practical syllogism. Thus, e.g., synderesis says: « Evil is to be shunned. » Reason subsumes: « Yet adultery is evil. » And conscience concludes in the particular: « Therefore adultery is to be shunned by me here and now. »

The definition is explained. It is called judgment or dictate, because conscience is an act, not a power or habit; for a power cannot be laid aside, and a habit is hard to move, but conscience can be laid aside and easily changed. It is called judgment or dictate of the intellect, because conscience pertains to the intellect, not to the will, as appears from the very etymology of the name; for conscience, as I said, is cum alio scientia. It is called judgment of the practical intellect, because the intellect, as while occupied by moral science with general conclusions concerning the goodness or malice of human actions, is called speculative-practical; so while occupied through conscience with particular conclusions concerning the goodness or malice of this act to be placed or shunned here and now, is called practice-practical.

It is called judgment, not imperium, which is expressed in words of the imperative mood: « Do this; » because an imperium of this kind supposes conscience, and is often disconformable to it, as when conscience dictates that something be shunned as evil, and man, reason commanding and conscience remorseful, does it.

It is called dictating something in particular to be done or shunned ² ; because although conscience be also of past things by testifying, approving, reproving, remorsing — of which Apostle, II Cor., I, 12 : This is our glory, the testimony of our conscience; and Isa., LXVI, 24, concerning the conscience of the damned: Their worm shall not die — therefore conscience in all its breadth ought to have been defined, « dictating something to be, to be about to be, or to have been done or shunned. » Because however we treat here of conscience as rule of things to be done, therefore we define under the limitation placed.

[ Conscience differs from Synderesis, which is not a practice-practical judgment concerning things to be done here and now, but « the habitus of operable principles, » which habitus also pertains to the intellect. The Angelic Doctor thus determines the nature of synderesis, Ia p., q. 79, a. 12, c. : « Synderesis is not a power, but a habitus: although certain ones placed synderesis as a certain power higher than reason, others indeed said it is reason itself, not as it is reason, but as nature. For the evidence of this it must be considered that man's ratiocination, since it is a certain motion, proceeds from the understanding of certain things, namely naturally known without investigation of reason, as from a certain immobile principle, and terminates at understanding also, inasmuch as we judge through principles per se naturally known concerning the things found out by ratiocinating. But it stands established that, as speculative reason ratiocinates about speculative things, so practical reason ratiocinates about operable things. There must therefore be naturally indited in us, as principles of speculative things, so also principles of operables. But first, the principles of speculative things naturally indited in us do not pertain to any special power, but to a certain special habitus, which is called the understanding of principles, as appears in VI Ethic., c. 6. Whence also the principles of operables naturally indited in us pertain not to a special power, but to a special natural habitus, which we call synderesis. Whence synderesis is said 'to instigate toward good and murmur about evil,' inasmuch as we proceed through first principles to find out, and judge what is found. It appears therefore that synderesis is not a power, but a natural habitus. »

Synderesis therefore is habitus, but conscience act.

Through synderesis the intellect possesses general principles whose light we can judge of the morality of actions, and conscience is an act of the intellect commanded by the will whereby we apply moral principles to judging of the morality of our actions in the singular.

Sometimes however synderesis is named conscience, but improperly, because habitus is taken for the act of which it is principle. Which our Doctor marks ibidem, a. 13, c. : « Properly speaking, conscience names an act. But since habitus is the principle of act, sometimes the name of conscience is attributed to the first natural habitus, namely synderesis, as Jerome, in glossa Ezech. I, calls synderesis conscience, and Basil, natural judgment, and Damascene says that it is the light of our intellect. For it is customary that causes and effects be mutually named. » ]

I say 2° : Conscience is divided commonly into true and false or erroneous, uncertain and probable or opinionative, and doubtful; into perplexed, scrupulous and lax; because reason judges in just so many ways from common principles of the licitness or illicitness of a particular action, as will appear from the enumeration and explanation of each.

True conscience is that which from true principles dictates something in particular to be licit or illicit which is really such.

False or erroneous conscience is that which from false principles, nevertheless reputed true, dictates something to be licit or illicit which is not really such. And it is twofold, vincibly and invincibly erroneous, like ignorance itself.

Certain conscience is that which formally and in itself, and in all the principles from which it is immediately formed, is such as to exclude every reasonable fear of the opposite.

Probable or opinionative conscience is that which from merely probable principles dictates something to be licit or illicit, with fear of the opposite part.

Doubtful conscience is the suspension of the intellect assenting to neither part of a contradiction, either because of contrary reasons equally probable, and it is called doubtful positively; or because it has no reasons pro or contra, and is called doubtful negatively. It would better be called un-knowing or ignorant. For one asking whether the sands of the sea are equal, one would better answer: « I know not, » than: « I doubt; » because if asked further why I doubt, I should have nothing to answer. Conscience doubtful in either way does not seem conscience properly and strictly so called, because it is not a definite judgment, such as conscience ought to be.

[ Perplexed conscience is that whereby someone, constituted amid two precepts, believes he sins whichever part he choose. E.g., if one could by perjury save the life or liberty of the accused in court, and were distressed on the one side by the precept of religion not to perjure himself, and on the other, led by error, by the precept of charity toward his neighbor, nor could resolve the difficulty. ]

Scrupulous conscience is that which from futile and utterly contemptible reasons judges or fears lest an action be evil. I say: « Judges or fears, » because if it truly from these light and vain reasons judge the action evil, it is truly conscience; but if it only fear lest it be evil against the judgment it has of its goodness, it is not properly conscience, but rather the shadow or likeness of conscience. The first is called scruples formed, the second scruples agitated. Against the former it is not lawful to act, well against the latter, as I shall say below.

Lax or loose conscience is that which from very slight and contemptible moments judges that not evil which is really evil, or a light evil which is grave, when either can easily be detected. To this approaches the pharisaical conscience, which makes great things small and small things great, straining out the gnat and swallowing the camel, Matth., XXIII, 24. These two consciences, scrupulous and lax, differ in this, as appears, from the probable, that the probable rests upon weighty moments, not these; whence they belong to the erroneous.