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I was speaking in generalities.  Logic.  Science.  Facts.  Take your pick.
An outcome cannot be certain when an input is presumed.

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Presumed=converted to morally certain.
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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."
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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. 
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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. 

I guess you could say there are two intentions that need to be formed.  One is the intention to form the correct intention.  And the other is to perform the ceremony correctly.  If the ceremony is performed correctly then it is certain that the intention was also correct.  The only way the intention could not have been correct was if there was an external sign.

Jaynek cited Cardinal Billot on another thread:

The intention of doing what the Church does, whatever that may be in 
the opinion of him who administers the sacrament, is said to be required. 
Thus St. Thomas: "Although he who does not believe that baptism is a 
sacrament, or does not believe that it has any spiritual power, does not 
intend when he baptizes to confer a sacrament, nevertheless he intends to 
do what the Church does, even if he counts that as nothing; and because the 
Church intends to do something, therefore, as a consequence of this, he 
intends implicitly to do something, though not explicitly."[1] But it is 
not necessary that the minister think as the Church does, or that he not 
err concerning her teaching; for it is enough if his intention is towards 
something which is identically that which the Church intends, or, something 
which amounts to the same thing, for example, if he intends to do that 
which Christ instituted, or which is commanded in the Gospel, or which 
Christians are accustomed to do according to the prescription of their 
religion. (Thus it is apparent how even a Jew or a pagan can have an 
intention sufficient for baptizing. Consider for example a catechumen 
placed in a moment of necessity, who asks a pagan saying, "Do for me, I 
entreat you, this mercy, that you pour water on me, pronouncing the words, 
'I baptize you,' etc., with the intention of doing what I myself intend to 
receive according to the prescription of the law of Christians.)