Then how does your post help answer the question?
Thanks for clarifying…I’m sorry that I am not being clear. I’ll try to do better.
We all know that what the definition of mortal sin is. What we don’t know is how it comes about that we really do get full knowledge of anything?
To answer this question we must place parameters on what constitutes “full knowledge” within the context of a given discussion. Otherwise, philosophically, one is left with the very conundrum you pose here. Thankfully in the case of sin, we have concrete parameters set for us by the Church.
In regards to matters of sin - we have a “specification” in which the question of “full knowledge” is spelled out for us
So - To answer your first question:
Q:…
“how it comes about that we really do get full knowledge of anything?”
A:…One looks up the definition of “full knowledge” within the context and documentation in which it is being used.
In the case of sin…Where we are guided by the catechism…
The answer is:
Full knowledge is understood to exist where one has)…“knowledge of the sinful character of the act, of its opposition to God’s law.” (CCC-1859)
The Catechism goes further and defined “God’s Law” (or the summary thereof) as the ten commandments. (CCC-577, 2058).
Thus - according to the Catechism - we get to “full knowledge” of what is sinful by knowing the Ten commandments.
and in any case, how we can decide between two not fully known proposed courses of action?
This question can certainly be a bit trickier. I cannot say that the catechism directly deals with this issue…perhaps someone else can enlighten us both on that.
But - my thought upon reading this question was to look directly at Scripture for guidance. Specifically in two places.
First, in Mt 22:36-40, Jesus summarizes all of the law and all of the prophets into two commandments. Love God above all else, and Love your neighbor as yourself.
Second, in 1 Cor 10:23, St Paul points out that, “All things are lawful,” but not all things are helpful. All things are lawful," but not all things build up".
Now in a very real sense, Paul is speaking pretty clearly to what you are asking…what to do when things are not “fully known”. At that time there was much discussion about whether the Mosaic Laws were still binding or not. This posed certain problems due to dietary restrictions etc and Paul was trying to address this by providing guidelines rather than rules.
What is interesting in this is how Paul’s guidelines flow back to Jesus summary of all of the law and prophets. That Love (Agape) must exist at the core of these matters and one should choose the most loving course in a given situation.
I hope the above helps. If not…well keep asking and we’ll try to sort things out…
Peace
James