ther it belong
to the substance of the sin, or not; and nevertheless his knowledge
is sufficient for him to be aware that the act is sinful; for
instance, if a man strike someone, knowing that it is a man (which
suffices for it to be sinful) and yet be ignorant of the fact that it
is his father, (which is a circumstance constituting another species
of sin); or, suppose that he is unaware that this man will defend
himself and strike him back, and that if he had known this, he would
not have struck him (which does not affect the sinfulness of the
act). Wherefore, though this man sins through ignorance, yet he is
not altogether excused, because, not withstanding, he has knowledge
of the sin. Secondly, this may happen on the part of the ignorance
itself, because, to wit, this ignorance is voluntary, either
directly, as when a man wishes of set purpose to be ignorant of
certain things that he may sin the more freely; or indirectly, as
when a man, through stress of work or other occupations, neglects to
acquire the knowledge which would restrain him from sin. For such
like negligence renders the ignorance itself voluntary and sinful,
provided it be about matters one is bound and able to know.
Consequently this ignorance does not altogether excuse from sin. If,
however, the ignorance be such as to be entirely involuntary, either
through being invincible, or through being of matters one is not
bound to know, then such like ignorance excuses from sin altogether.

Reply Obj. 1: Not every ignorance causes involuntariness, as stated
above (Q. 6, A. 8). Hence not every ignorance excuses from sin
altogether.

Reply Obj. 2: So far as voluntariness remains in the ignorant person,
the intention of sin remains in him: so that, in this respect, his
sin is not accidental.

Reply Obj. 3: If the ignorance be such as to exclude the use of
reason entirely, it excuses from sin altogether, as is the case with
madmen and imbeciles: but such is not always the ignorance that
causes the sin; and so it does not always excuse from sin altogether.
________________________

FOURTH ARTICLE [I-II, Q. 76, Art. 4]

Whether Ignorance Diminishes a Sin?

Objection 1: It would seem that ignorance does not diminish a sin.
For that which is common to all sins does not diminish sin. Now
ignorance is common to all sins, for the Philosopher says (Ethic.
iii, 1) that "every evil man is ignorant." Therefore ignorance does
not diminish sin.

Obj. 2: Further,