Is it impossible for a finite human to commit a mortal sin by its very definition?

I've only read half the introduction to Peter Kreeft's Socratic Logic so bear with me, I'm finally beginning to understand the distinction between the human logic of Aristotle and the symbolic logic of the computer that I'm acquainted with as a programmer.

Aristotelian logic accepts the qualifiers as theorems and if they're true - whatever they add up to is true. The Church admits them theorems like: "all men are cads" and "some sins are worse than others" and "people have freewill" and "some consciences are well formed"

These form the argument "because all men are cads they may want to commit sins and because they have free will they may choose to commit those sins and because some sins are worse than others, they may be classified as mortal sins and because a persons conscience can be well formed they can objectively discern that they committed a mortal sin; therefore it is possible for a person to commit a mortal sin."

What I'm not sure that it is possible for a person to do is to be free from an attachment to sin (a pre-requisite for a plenary indulgence) but I'd like to think we can try and God will respect our efforts, finite as they may be.

The thesis here is that there is no sin where the sin is known to be grave and fully consented to. Because of this, the question is how can one mortally sin

The can be easily disproved using the Catholic Church's requirement to attend Mass on Sundays and obligatory Holy Days as an example. Here, the Church has bound the faithful to such obligation under pain of mortal sin. As long as a Catholic thinks that the Church has that power on Earth, given by Christ, then it follows that this constitutes grave matter. Furthermore, it is evident that we can know that this constitutes grave matter, as the preceding argument proves it.

All that remains is the ability to fully consent, and it is manifestly evident that as long as the concept of consent is useful, one can consent to missing Mass on one of these obligatory days. If one cannot fully consent because of some unknowable subconscious processes, then the idea of consent used in so many other places becomes incoherent. In business contracts, in criminal law, in divorce courts, etc. The only way to hold that one cannot consent to skipping mass is to say that no one can ever really consent to anything.

QED, at least with this one particular sin, it is possible to sin mortally.

Fallible humans can definitely fulfil the conditions to commit a mortal sin.

Let's take a really clear cut case. Let's say I embezzled some money from my employer. A co-worker discovers this and tells me they will go to the police. Before they can do this I put a gun to their head and kill them.

Let's look at the conditions:

1. Grave matter. It is clear from the teachings of the church that killing someone is a grave matter. It ought to be obvious to anyone that is the case, even without the church's teaching. There is no reasonable way I can claim i thought killing someone was not "grave".
2. Full knowledge. I am aware of the pertinent facts. I know that when I pull the trigger they will die. I know that they are a human. I have no reason to think that killing them is justified. Even if I don't have omniscient knowledge of the circumstances I have enough knowledge to know that what I do is wrong.
3. Deliberate consent. Nothing is forcing me to kill this person. Nobody has misled me into thinking that I ought to kill this person. Whatever pressures or temptations I have to kill someone I have the choice to do it or not, and if I do it with thought then I give my deliberate consent.

Even if it is true that there are "subconscious urges" or anything else that "contribute" to me wanting to kill this person, if I actually have the choice to do it or not then the decision is mine and that amounts to deliberate consent.

If I am suffering from something like a mental disorder that effectively removes freedom of choice from me that it may be true that I have not sinned. Mitigating factors that make it hard for me to resist sinning might also reduce the perceived severity of an offence. But those mitigating factors may or may not be there, and short of a disorder everybody has a choice in what they do.

To answer the question a posed: "Yes it is absolutely possible for a person to commit a mortal sin."