Is this a mortal sin?

If a person was daydreaming–mind wandered, not being aware of the gravity of the daydream-- about committing a mortal sin, that is, within the day dream there was full knowledge consent and grave matter about the act, but once that person outside the daydream realized something was seriously wrong they stopped, was it a mortal sin? Or is that like mortally sinning while dreaming and not fully conscious of the act or maybe partial consent, and therefore no mortal sin? Is that a thought to be treated like other “impure thoughts” that one hesitates rejecting–the matter in question isn’t sexual in nature maybe why I’m more confused?

It sounds like you are speaking hypothetically, so I don’t want to assume you are actually the one having impure daydreams. But in general this is a matter to bring up with one’s confessor who has heard plenty of these cases and probably knows how to get to the root. One could be suffering from scrupulosity, in which case I recommend this: Especially this one:

  1. You shall not consider yourself guilty of bad thoughts, desires, or feelings, unless you can honestly swear before the all-truthful God that you remember clearly and certainly consenting to them.

This is a very important commandment. The whole area of impure thoughts and desires causes scrupulous people much anxiety. Unfortunately, scrupulous persons often believe that the very appearance of thoughts or desires in their thoughts or imagination means that they have committed a sin. This is most certainly not the case. In fact, it is humanly impossible for us to have absolute control over our interior faculties. Such thoughts and images are going to happen, whether we like them or not.

Because we simply do not have absolute control over our interior faculties, the emphasis of the commandment is on clear and certain consent. Only a free consent, that is clear and certain, constitutes a sin. You can not accidentally or involuntarily be guilty of sin.

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