I was speaking in generalities. Logic. Science. Facts. Take your pick.
An outcome cannot be certain when an input is presumed.
.
Presumed=converted to morally certain.
.
To say intention is presumed is
not to say "well, I'll never know, I'll just
presume he intends to make a sacrament happen." To say intention is presumed is to say "Given that men act as they think, and this man is acting in the only way by which it is possible to confect a given sacrament, I
presume he
intends to act this way, and
therefore am morally certain that he has the required intention."
.
But to take the "invisibility" of intention and, from that, conclude we cannot
be (morally) certain about intention is wrong. If we are
not morally certain about intention, then we can't partake in the sacrament.
.
The reason that the language is that we
presume intention is that intention is internal. We can actually
see/hear that matter and form are correct and present, so there is nothing to
presume in regard to those elements. We cannot
see intention, we can only see indicators of it (mainly, the presence of matter and form correctly applied). But in the presence of those indicators, we
presume that the intention is there, and that presumption
itself is controvertible with moral certainty. So don't let the language throw you off.