פירוש על קידושין 99:9

The Talmud now tries to find other responses the master (the one who sends the agent) could have said to exempt himself from a sacrifice. The fact that he did not state these excuses means that he is telling the truth when he says “I was thinking.” And if we nevertheless don’t believe him, it can only because “matters of the heart are not significant.”
A person who intentionally commits trespass is not liable for a sacrifice. So while theoretically he could have said “I intentionally sent him to take sacred property” people do not want to say that they would intentionally commit a sin.
However, he could have said “I remembered” that the property was holy after I sent him to get it. Since he could have said this and thereby exempted himself for a sacrifice, but did not say this, he did not need to say “I was thinking [that it was not holy].” Nevertheless, we still do not believe him. This proves Rava’s rule that matters of the heart are not significant.