HINT:  this internal vs. external distinction is also confounded by people who equate the obligation to give "internal assent" to even he on-infallible teachings of the Church with internally believing the truth (even if you think it's wrong).  There's an analogy with the warping of that term with the warping of "internal intention" vis-a-vis the Sacraments.

Since the slanderer will undoubtedly be stumped and have no answer, "external intention" refers to

MERELY HAVING THE INTENTION TO PERFORM THE EXTERNALS

I saw someone pouring water on a person's head while saying the words "I baptize thee etc."  Yep.  Valid.  But, wait a minute ... this was in a movie where a guy was dressed like a priest.  Doesn't matter.  Valid. [=external intention, i.e. it suffices to intend to perform the externals only].  But, wait a minute ... I saw the guy running around laughing and sticking his tongue out, saying, "haha, I baptize thee etc. ... you moron".  Doesn't matter.  Valid [=external intention].  But, wait a minute ... the priest said he was just demonstrating how to perform the Rite.  Doesn't matter.  Valid [=external intention].

THAT is what's meant by "external intention", and we understand "internal intention" by contrast with this term.  Simply intending to perform the externals of the Rite, saying the right words, pouring the water (or applying other matter) ... does to not suffices by themselves.  Those must be accompanied by the intention of the minister to be doing what the Church does and acting in the capacity of a minister of the Church.

Catharinus' position might only exclude someone who was, say, sleepwalking and had no idea what he was saying and yet performed a Sacrament, since there was no intention there whatsoever.