(b) It is also lawful to read the writing of others that are not secret as regards oneself, as when one has received a just permission from the writer to peruse a letter written by him, or when one may presume such permission on account of friendship with the writer, or when rule or lawful custom gives the superior of an institution the right to inspect the correspondence of his subjects. Exception must be made for exempted matter for which there is no permission, such as letters containing conscience matters and letters directed to higher religious superiors (see Canon 611).
(c) It is also lawful to read the writings of others that are meant to be secret, if one has a right to know what is in them: for in such a case the owner would be unreasonable if he wished to exclude one from the knowledge. Thus, the public authority (e.g., in time of war) has the right to open and read letters and private papers, when this is necessary for the common good; parents and heads of boarding schools may examine the correspondence of their subjects, though parents should respect conscience matter and others should not read family secrets; private individuals have the natural right, as a measure of self-defense, to read another’s letter, when there is a prudent reason for thinking that it contains something gravely and unjustly harmful to themselves (such as conspiracy, a trap, calumny).
2412. Lawfulness of Utilizing Knowledge of Secret.—One is said to use the knowledge obtained from a secret when one guides one’s conduct by the knowledge, doing or omitting what one would not otherwise do or omit. Is this use of a secret lawful?
(a) If there was a promise not to use the secret, such use is unlawful (see 2414). Breach of promise is then, in case of a merely promised secret, an act of infidelity at least, and in case of an entrusted secret an act of injustice. Thus, when one consults a professional person, there is a tacit understanding that the knowledge communicated will not be used against one’s interests or without one’s consent, and hence a lawyer would be unjust if, on learning in the course of work for a client that the latter’s business was not prosperous, he gave word of this to one of the client’s creditors.
(b) If there was no promise not to use the secret, the use of it is nevertheless unjust, if it infringes a strict right (e.g., to make money from a secret process on which another has a patent, to get knowledge of another’s information and plans through reading his letters and thereby to prevent him from securing a vacant position), or if it is equivalent to unjust revelation of a secret. The use is uneharitable if it harms another person without necessity (e.g., to take away one’s trade from a deserving merchant solely because one has learned that on one occasion he was accidentally intoxicated).
(c) If there was no promise to avoid use and no harm will be done by use, it is lawful to use a secret for a non-necessary good (e.g., to raise the price on one’s property when one accidentally learns through overhearing a secret conversation that the property is worth the higher price), and it is obligatory to use it for a necessary good (e.g., to assist a neighbor when one is told under secret that he is in dire need of one’s charitable help). Even though harm will result to another by use of the secret, use is not sinful if it infringes no right and could be sacrificed only at great inconvenience to oneself, as when one has discovered by one’s own industry some important truth in an art or science which another had previously discovered but had neglected to make his own by exclusive right, or when one learns under secret that another person is one’s enemy and has to be watched and avoided.
2413. The Sin Committed by Stealing or Unduly Using the Secret of Another.—(a) From its nature (cases of mere fidelity excepted) the sin is mortal, as being a violation of commutative justice or of charity. Injury to property rights, whether in goods or in knowledge, is violation of a strict right (see 1890, 1894). The sin is aggravated by the greater import of the secret or by the greater damages or displeasure caused.
(b) From the imperfection of the act or the lightness of the matter the sin may become venial, as when one thoughtlessly reads another person’s letters, or opens correspondence without authority, feeling morally sure that there is nothing confidential in it, or makes use of an unimportant secret without permission.
2414. The Obligation of Keeping a Secret.—(a) The natural secret obliges _per se_ under grave sin; for violation of it offends charity and justice by saddening and harming a neighbor. The sin may become venial on account of lightness of matter, as when little sadness or harm is caused.
(b) The promised secret obliges ordinarily under light sin only; for as a rule the promisor intends to obligate himself in virtue of fidelity alone (1888), and the obligation of fidelity, as said above (see 2407), is not grave. But exceptionally the obligation may be grave, as when the promisor intended to bind himself in virtue of justice and under grave sin, or when the secret is natural as well as promised.