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St. Thomas Aquinas
Catechetical Instructions

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  • THE EIGHTH COMMANDMENT: "Thou Shalt Not Bear False Witness Against Thy
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The prohibition of this Commandment includes every form of falsehood: "Be

not willing to make any manner of lie; for the custom thereof is no

good."17 There are four reasons for this. The first is that lying likens

one to the devil, because a liar is as the son of the devil. Now, we know

that a man's speech betrays from what region and country he comes from,

thus: "Even thy speech doth discover thee."18 Even so, some men are of the

devil's kind, and are called sons of the devil because they are liars,

since the devil is "a liar and the father of lies."19 Thus, when the devil

said, "No, you shall not die the death,"20 he lied. But, on the contrary,

others are the children of God, who is Truth, and they are those who speak

the truth.


The second reason is that lying induces the ruin of society. Men live

together in society, and this is soon rendered impossible if they do not

speak the truth to one another. "Wherefore putting away Iying, speak ye the

truth, every man with his neighbor; for we are members one of another."21


The third reason is that the liar loses his reputation for the truth. He

who is accustomed to telling lies is not believed even when he speaks the

truth: "What can be made clean by the unclean? And what truth can come from

that which is false?"22


The fourth reason is because a liar kills his soul, for "the mouth that

belieth killeth the soul."23 And again: "Thou wilt destroy all that speak a

lie."24 Accordingly, it is clear that lying is a mortal sin; although it

must be known that some lies may be venial.


It is a mortal sin, for instance, to lie in matters of faith. This concerns

professors, prelates and preachers, and is the gravest of all other kinds

of lies: "There shall be among you lying teachers, who shall bring in sects

of perdition."25 Then there are those who lie to wrong their neighbor: "Lie

not to one another."26 These two kinds of lies, therefore, are mortal sins.


There are some who lie for their own advantage, and this in a variety of

ways. Sometimes it is out of humility. This may be the case in confession,

about which St. Augustine says: "Just as one must avoid concealing what he

has committed, so also he must not mention what he has not committed."

"Hath God any need of your lie?"27 And again: "There is one that humbleth

himself wickedly, and his interior is full of deceit; and there is one that

humbleth himself exceedingly with a great lowness."28


There are others who tell lies out of shame, namely, when one tells a

falsehood believing that he is telling the truth, and on becoming aware of

it he is ashamed to retract: "In no wise speak against the truth, but be

ashamed of the lie of thy ignorance."29 Other some lie for desired results

as when they wish to gain or avoid something: "We have placed our hope in

lies, and by falsehood we are protected."30 And again: "He that trusteth in

lies feedeth the winds."31


Finally, there are some who lie to benefit another, that is, when they wish

to free someone from death, or danger, or some other loss. This must be

avoided, as St. Augustine tells us: "Accept no person against thy own

person, nor against thy soul a lie."32 But others lie only out of vanity,

and this, too, must never be done, lest the habit of such lead us to mortal

sin: "For the bewitching of vanity obscureth good things."33






17. Ecclus, vii. 14.


18. Matt., xxvi. 73.


19. John, viii. 44.


20. Gen. iii. 4.


21. Eph., iv. 25.


22. Ecclus., xxxiv. 4.


23. Wis., i. 11.


24. Ps. v. 7.


25. II Peter, ii. 1.


26. Col., iii. 9.


27. Job, xiii. 7.


28. Ecclus., xix.


29. "Ibid.," iv. 30.


30. Isa., xxviii. 15.


31. Prov., x. 4.


32. Eccles., iv. 26.


33. Wis., iv. 12.

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