On the Sixth and Ninth Precept of the Decalogue

You shall not commit adultery. - You shall not desire his wife (that is, your neighbor's). - Exod. xx, 14

The Sixth Precept of the Decalogue prohibits any external lust, that is, lust perpetrated by deed. - But the Ninth Precept prohibits even merely internal lust, namely impure thoughts and desires.

For the understanding of those things which follow, these things are to be noted beforehand:

I. Lust is the disordered appetite of a venereal thing, and consists in venereal pleasure voluntarily admitted outside marriage. But venereal pleasure is pleasure arising from the stirring of the spirits serving generation, and occurs around the genitals. Therefore it is not to be confused with merely sensible pleasure, which arises from the proportion of a sensible object to some sense, e.g., of a visible object to sight, of an audible object to hearing, etc. The first, many theologians call also carnal, and the other natural.

II. There are various species of lust, according as its objects are diverse. Thus impure touches and gazes differ in species; solitary acts and acts exercised with others; consummated and non-consummated acts; likewise acts consummated according to nature and acts consummated against nature. But these species contain under themselves other subordinated or inferior species also, as will be clear from what is to be said.

III. One lust is directly sought, another indirectly. The former is had when carnal pleasure is intended precisely in itself; but the latter when something else is intended, from which carnal pleasure arises beyond the intention, as happens in less chaste gazes or readings had from curiosity or some other end. These things having been noted beforehand:

I say 1st. Every venereal pleasure, if it is directly voluntary, does not admit smallness of matter, and accordingly, however slight it may be, it will always be a mortal sin. The reason is that venereal pleasure is a certain begun emission of semen, or a natural and necessary motion toward it: but emission of semen, if it is sought outside the legitimate use of marriage, is a grave disturbance of the essential order, since from its own nature it tends to the destruction of the human race. Therefore. I said if it is directly voluntary, for all concede that smallness of matter can be given in venereal pleasure only indirectly willed, and the reason is evident; for the whole malice of this kind of pleasure must be taken from the cause; but that there can be a cause slightly base, e.g., curious reading of some less chaste book, no one has ever denied. - Cf. St. Alphonsus, nn. 415 and 484. - Viva, on prop. 40 condemned by Alexander VII. Baller., note (a), p. 396.

I say 2nd. Pleasure which is called sensible or natural of itself lacks every fault. For that pleasure properly consists in this, that the touch or sight of one's own body or another's pleases on account of proportion and connaturality with the organ of touch or sight, without any stirring of the genital spirits; not otherwise than if sometimes a soft thing were touched, e.g., a cat, silk, etc., or a beautiful thing were looked at, e.g., a horse, a meadow, and other things which naturally delight: but in touching or looking upon these things there is or can be no fault of itself. Therefore. I said of itself, for practically speaking there will always be some slight fault present, not only if those touches or gazes are done from curiosity, levity, or for the sole pleasure, according to those things which have been said elsewhere, but also on account of the danger of passing over to venereal pleasure. - Cf. St. Alphonsus, n. 416. Viva, loc. cit. - Baller., note (a), p. 393.

Now one must treat of the sins of lust in particular, but only of external ones, because of internal ones enough has been said elsewhere.