c) Such acts should not too quickly be regarded as light matter. Like most other injustices, many forms of wrongful taking can admit parvity of matter. Casuists have discussed this question so often that legalistically minded thieves may be able to find opinions which seem to treat what they are doing as light matter. Before judging that one can engage in such an injustice without committing a mortal sin, however, a person should bear several things in mind.
First, the matter is not light if it is likely to cause serious harm to anyone. For example, stealing a very small amount from a child or a poor person can cause great distress; taking something of little value in itself can lead to considerable trouble and inconvenience. The matter cannot be judged light if a reasonable person in the circumstances of any individual harmed would consider the harm significant, that is, would be noticeably saddened or angered by harm of that kind and amount (see S.t., 2–2, q. 66, a. 6, ad 3). In evaluating the significance of harm suffered, however, one may take into account that a reasonable person might be less attached to property than some owners in fact are. For example, a wealthy individual of a miserly disposition might be very distressed if it appears that a cashier has shortchanged him or her by one dollar.
Again, even if an act or practice of wrongful taking does not harm any one person significantly, it may be gravely unjust because of its effect on the community as a whole. For example, petty cheating by some merchants in classifying, adulterating, weighing, and measuring goods can render honest competition impossible. Similarly, petty shoplifting can increase the costs of doing business so greatly that it cannot reasonably be considered insignificant; a sign of this is that public opinion generally supports the enforcement of laws forbidding it.
Given that an act is wrongful taking, doubts about its seriousness should be settled by presuming the matter grave. Someone who is willing to do wrong while doubtful about how serious the wrong will be, is willing to do the more serious wrong (see 5.I.2.a).