(b) External dangers to faith are especially as follows: literature opposed to religion; schools where unbelief is defended; mixed marriages; association with unbelievers in religious matters; certain societies. 849. Dangerous Reading.--There is a threefold prohibition against the reading of literature dangerous to faith. (a) The natural law forbids one to read or hear read written matter of any description which one knows is dangerous to one’s faith, even though it is not dangerous to others and not forbidden by the law of the Church. For a similar reason one may not keep such material in one’s possession. Example: Titus and Balbus read the letters of a friend on Evolution. Titus finds nothing unsound in the letters, and is not troubled by reading them; but they fill the mind of Balbus with doubts and perplexities, as the subject is above him. This reading is naturally dangerous for Balbus, but not for Titus. (b) The law of the Church forbids the use of certain kinds of writings or representations dangerous to faith (Canon 1399), as well as of those individual writings that have been denounced to the Holy See and placed on the Index, or forbidden by other ecclestiastical authorities. (See Appendix I for Summary of Common Law on Prohibition of Books.) (c) The law of the Church also pronounces ipso facto excommunication against those who make use of works written by unbelievers in favor of their errors (Canon 2318). 850. As regards the kind of sin committed by using writings dangerous to faith, the following points must be noted: (a) If a writing is dangerous and forbidden under natural law, the sin committed is of itself grave whenever the danger itself is serious and proximate; it is venial, when the danger is slight or remote. The sin committed depends, therefore, not on the time spent in reading or the number of pages covered, but on the danger (see 260-261, on the Dangers of Sin). No sin at all is committed, if the danger is slight or remote, and there is reason for reading the writing in question (e.g., the defense of truth). (b) If the writing is forbidden under ecclesiastical law, the sin committed is of itself grave, even though the danger to an individual is not serious or proximate, for the law is based on the presumption of a common and great danger (see 460). The sin is not grave, however, when the prohibition is generally regarded as not binding under grave sin, or when the use made of the writing is inconsiderable. No sin at all is committed, if one has obtained the necessary permission to read forbidden works, and is not exposed to spiritual danger in using the permission.