in either of two things:  first, in the thought itself, secondly, in the fornication thought of.  Now the delectation in the thought itself results from the inclination of the appetite to the thought; and the thought itself is not in itself a mortal sin; sometimes indeed it is only a venial sin, as when a man thinks of such a thing for no purpose; and sometimes it is no sin at all, as when a man has a purpose in thinking of it; for instance, he may wish to preach or dispute about it.  Consequently such affection or delectation in respect of the thought of fornication is not a mortal sin in virtue of its genus, but is sometimes a venial sin and sometimes no sin at all:  wherefore neither is it a mortal sin to consent to such a thought.  In this sense the first opinion is true.

But that a man in thinking of fornication takes pleasure in the act thought of, is due to his desire being inclined to this act.  Wherefore the fact that a man consents to such a delectation, amounts to nothing less than a consent to the inclination of his appetite to fornication:  for no man takes pleasure except in that which is in conformity with his appetite.  Now it is a mortal sin, if a man deliberately chooses that his appetite be conformed to what is in itself a mortal sin.  Wherefore such a consent to delectation in a mortal sin, is itself a mortal sin, as the second opinion maintains.

Reply Obj. 1:  Consent to delectation may be not only in the lower reason, but also in the higher reason, as stated above (A. 7).  Nevertheless the lower reason may turn away from the eternal types, for, though it is not intent on them, as regulating according to them, which is proper to the higher reason, yet, it is intent on them, as being regulated according to them:  and by turning from them in this sense, it may sin mortally; since even the acts of the lower powers and of the external members may be mortal sins, in so far as the direction of the higher reason fails in directing them according to the eternal types.

Reply Obj. 2:  Consent to a sin that is venial in its genus, is itself a venial sin, and accordingly one may conclude that the consent to take pleasure in a useless thought about fornication, is a venial sin.  But delectation in the act itself of fornication is, in its genus, a mortal sin:  and that it be a venial sin before the consent is given, is accidental, viz. on account of the incompleteness of the act:  which incompleteness ceases when the deliberate consent has been given, so that therefore it has its complete nature and is a mortal sin.

Reply Obj. 3:  This argument considers the delectation which has the thought for its object.

Reply Obj. 4:  The delectation which has an external act for its object, cannot be without complacency in the external act as such, even though there be no decision to fulfil it, on account of the prohibition of some higher authority:  wherefore the act is inordinate, and consequently the delectation will be inordinate also.