Objection 1. It would seem that scandal is unfittingly defined as "something less rightly said or done that occasions spiritual downfall." For scandal is a sin as we shall state further on (2). Now, according to Augustine (Contra Faust. xxii, 27), a sin is a "word, deed, or desire contrary to the law of God." Therefore the definition given above is insufficient, since it omits "thought" or "desire."
Objection 2. Further, since among virtuous or right acts one is more virtuous or more right than another, that one alone which has perfect rectitude would not seem to be a "less" right one. If, therefore, scandal is something "less" rightly said or done, it follows that every virtuous act except the best of all, is a scandal.
Objection 3. Further, an occasion is an accidental cause. But nothing accidental should enter a definition, because it does not specify the thing defined. Therefore it is unfitting, in defining scandal, to say that it is an "occasion."
Objection 4. Further, whatever a man does may be the occasion of another's spiritual downfall, because accidental causes are indeterminate. Consequently, if scandal is something that occasions another's spiritual downfall, any deed or word can be a scandal: and this seems unreasonable.
Objection 5. Further, a man occasions his neighbor's spiritual downfall when he offends or weakens him. Now scandal is condivided with offense and weakness, for the Apostle says (Romans 14:21): "It is good not to eat flesh, and not to drink wine, nor anything whereby thy brother is offended or scandalized, or weakened." Therefore the aforesaid definition of scandal is unfitting.
On the contrary, Jerome in expounding Matthew 15:12, "Dost thou know that the Pharisees, when they heard this word," etc. says: "When we read 'Whosoever shall scandalize,' the sense is 'Whosoever shall, by deed or word, occasion another's spiritual downfall.'"
I answer that, As Jerome observes the Greek skandalon may be rendered offense, downfall, or a stumbling against something. For when a body, while moving along a path, meets with an obstacle, it may happen to stumble against it, and be disposed to fall down: such an obstacle is a skandalon.
In like manner, while going along the spiritual way, a man may be disposed to a spiritual downfall by another's word or deed, in so far, to wit, as one man by his injunction, inducement or example, moves another to sin; and this is scandal properly so called.
Now nothing by its very nature disposes a man to spiritual downfall, except that which has some lack of rectitude, since what is perfectly right, secures man against a fall, instead of conducing to his downfall. Scandal is, therefore, fittingly defined as "something less rightly done or said, that occasions another's spiritual downfall."
Reply to Objection 1. The thought or desire of evil lies hidden in the heart, wherefore it does not suggest itself to another man as an obstacle conducing to his spiritual downfall: hence it cannot come under the head of scandal.
Reply to Objection 2. A thing is said to be less right, not because something else surpasses it in rectitude, but because it has some lack of rectitude, either through being evil in itself, such as sin, or through having an appearance of evil. Thus, for instance, if a man were to "sit at meat in the idol's temple" (1 Corinthians 8:10), though this is not sinful in itself, provided it be done with no evil intention, yet, since it has a certain appearance of evil, and a semblance of worshipping the idol, it might occasion another man's spiritual downfall. Hence the Apostle says (1 Thessalonians 5:22): "From all appearance of evil refrain yourselves." Scandal is therefore fittingly described as something done "less rightly," so as to comprise both whatever is sinful in itself, and all that has an appearance of evil.
Reply to Objection 3. As stated above (I-II, 75, 2,3; I-II, 80, 1), nothing can be a sufficient cause of a man's spiritual downfall, which is sin, save his own will. Wherefore another man's words or deeds can only be an imperfect cause, conducing somewhat to that downfall. For this reason scandal is said to afford not a cause, but an occasion, which is an imperfect, and not always an accidental cause. Nor is there any reason why certain definitions should not make mention of things that are accidental, since what is accidental to one, may be proper to something else: thus the accidental cause is mentioned in the definition of chance (Phys. ii, 5).
Reply to Objection 4. Another's words or deed may be the cause of another's sin in two ways, directly and accidentally. Directly, when a man either intends, by his evil word or deed, to lead another man into sin, or, if he does not so intend, when his deed is of such a nature as to lead another into sin: for instance, when a man publicly commits a sin or does something that has an appearance of sin. On this case he that does such an act does, properly speaking, afford an occasion of another's spiritual downfall, wherefore his act is called "active scandal." One man's word or deed is the accidental cause of another's sin, when he neither intends to lead him into sin, nor does what is of a nature to lead him into sin, and yet this other one, through being ill-disposed, is led into sin, for instance, into envy of another's good, and then he who does this righteous act, does not, so far as he is concerned, afford an occasion of the other's downfall, but it is this other one who takes the occasion according to Romans 7:8: "Sin taking occasion by the commandment wrought in me all manner of concupiscence." Wherefore this is "passive," without "active scandal," since he that acts rightly does not, for his own part, afford the occasion of the other's downfall. Sometimes therefore it happens that there is active scandal in the one together with passive scandal in the other, as when one commits a sin being induced thereto by another; sometimes there is active without passive scandal, for instance when one, by word or deed, provokes another to sin, and the latter does not consent; and sometimes there is passive without active scandal, as we have already said.
Reply to Objection 5. "Weakness" denotes proneness to scandal; while "offense" signifies resentment against the person who commits a sin, which resentment may be sometimes without spiritual downfall; and "scandal" is the stumbling that results in downfall.
Source: Summa Theologica (New Advent)